Pruning trees when dormant vs growing?

I've always pruned in late winter with the idea that it reduced chances for disease.  This year I started doing some pruning in the growing season because it let me head off undesirable growth before the tree pumped a lot of energy into it, and also gave me fodder for the livestock.  I really prefer the growing season pruning.  What do you do?  Will I regret it down the road?

not sure if it depend what kind of trees.
I learned from somewhere that peach trees should be pruned only in spring when they are flowering. and high bush blueberries all that's necessary is to clip off dead sticks and branches. but that's just my experience.

When to prune depends very much on the plant and on what you want to achieve.  In general, it's better for the plants themselves if you prune them in late winter/early spring, when it's started to warm up but the plants have not started to leaf out their spring growth. It's less stressful to them, their wounds heal faster and it directs growth to where you want it.  Prune in the summer if you want to retard the growth of the plant and make it smaller. However, you should stop pruning woody plants 6 to 8 weeks before you expect frost in order to allow the plant to heal and harden off properly before winter comes.
Another consideration is if and when the plant blooms and whether it blooms on old or new wood.  For example, rhododendrons bloom on old wood and set their buds in the fall for spring blooms.  Therefore, you'd probably want to avoid pruning them in the late winter/early spring because you want to keep the buds.  A good general rule is to prune blooming plants right after they bloom.
And yet another consideration is what pests you have in the area that may be invigorated by your pruning. Where I live, we avoid pruning pines at certain times of the year in order not to attract pine bark beetles.
Those are all general rules. I keep a chart of what plants I have and when I should prune them. Different plants have different requirements and my chart helps me not forget what and when I need to prune, so I am not forced into either pruning when I shouldn't or neglecting to prune a plant that year.

I've both heard and read about some genuine benefits of summer pruning. I wasn't as aware of this outside my ecosystem which is very rainy in the winter, thus a rather miserable time to be out pruning.

A friend talked me into buying an espalier pear tree for a spot that I didn't want a tree that would cast shade. The instructions specifically said to prune it in August and generally it has done well under that protocol. The issue is that despite my efforts, the deer keep breaking in and eating all the leaves. I have had to try and let the tree grow a little taller to keep some of the leaves out of reach.

I recently read Grow a Little Fruit Tree by Ann Ralph which advises summer solstice pruning to keep tree size in check (but not too close to fall as this type of pruning promotes growth, same as winter pruning). I did this (the solstice prune) for the last 2 summers and it worked well for the cherries that got too tall. I still winter prune for all the usual reasons. I tend to underprune anyway.

I do both now. Winter pruning for the structural stuff, removing crossing branches, shaping the canopy. Summer pruning to keep vigour in check on anything that's getting too tall or pushing growth where I don't want it. The summer cuts heal faster in my experience because the tree is actively growing, and you can see exactly what you're working with when everything is leafed out. Only thing I'd avoid is pruning stone fruit in winter because of the canker risk, those always get done in summer for me.

Someone at the allotments, an elderly man, who said he was very experienced with fruit trees, told me that there is no reason to prune in a certain season. He said something like 'prune whenever you want it'.

Now I do it like that. When I want to cut away a (part of a) branch, f.e. because it grows over a path, or it is too close to another branch, I just do it.

The general wisdom for pome and stone fruits is to strategically prune when dormant (late winter/very early spring before breaking dormancy, but during a warmer spell that stays above freezing for a few days) to encourage growth, because it stimulates the most growth hormone, and to prune in late summer to slow down growth because it doesn't cause the same level of hormone release.

But once they are out of dormancy, in many areas it's better not to prune pome fruits and to wait until closer to the fall, because pruning increases the risk of fireblight exposure in early to mid-summer. Some fruits like paw paws also apparently recover much better from pruning when they are growing, not dormant, so those specific types should only be pruned during the growing season.

But all of this wisdom really depends on the types of fruit, where you are located with your specific disease pressures, and your goals for each tree at that specific stage of growth (like you might want to be stimulating annual growth for the first few years, and then slow them down for easier management once they are bearing).

I prune in summer and winter.  One bad time to prune is when the sap is actively rising.  Too much life effort running out.  I don’t know for sure, but it also seems one more substance to attract and invite pests parasites and diseases.

I’m in agreement with the poster who said “it depends.”

I have posted about the ancient apricot orchard I once had.  The trees were so big that all the parts of the trees that could be easily harvested didn’t fruit, and all the fruit were out of reach.  Commercial orchards would have knocked them over long ago, but I had small children and that orchard was a magical playground!  Besides, the trees were so beautiful, provided wonderful shade and fruit I didn’t want to do that.

Rehabbing an old orchard is a legitimate reason to prune.  Think of it as a 3-5 year process

You want the sun light to reach lower down into the tree, because the buds for flower then fruit develop where the sunlight reaches.

You’ll end up removing large branches.

If you prune too much in the winter, then in the spring, you’ll get “water sprouts”. Also called witches’ brooms.  Too much sap rising for the number of branches, sprouts and buds to feed

Even if you don’t have the presence of fruit to guide you, wait for summer pruning until after the harvest.  

In winter I prune for shape and structure.  In the summer after harvest time and before the trees begin sending their sap down for the winter, prune for size, and the other things.

That’s apricots.  Apples fruit much later than apricots.  I have never had apple trees, so anything I might say would be conjecture.  It might have a high degree of accuracy, but it’s not the same as having had decades to study the effects of my efforts.

I would say, learn all you can about plants.  What is and where is the meristem.  What is the plant’s annual cycle.  What stimulates growth and what suppresses it.  What plant hormones exist and how do individual species  utilize them? How do rooted cuttings differ from seedlings…. And so on.  Assume plants are complex.  Learn all you can!  You need to know what the variables are to make good decisions, decisions that are going to bring about desired results.

And a caveat to make it a little harder:  not everyone who puts him or herself forward as knowledgeable is.  You need to know as much as possible about the teacher as you do about plants.  I have nothing against the master gardener programs and the extension offices, but, do they believe in pesticides and commercial fertilizers, soil amendments drip systems, whatever?  What do they consider “mulch”.  If they believe other than you do, then they can teach you plant physiology, but probably not the farming you want to do.

Surely there’s an Eastern Wisdom aphorism about the search for the teacher teaching the most valuable lessons.

If not, one is sure to pop up soon.

Happy pruning.