"If you are a perfect person that times everything you do perfectly..."
HAHAHAHAHAHA!
Oh, sorry, let me get back into my chair.
Here's my opinion, similar in some respects to Paul's:
Plant your seeds and cover with a very light mulch (dry
straw run through a shredder is perfect, or crumbled dry leaves -- between your hands is fine) just to help keep the soil surface damp so your seeds don't have to deal with that wet/dry/wet/dry stress. Once a sprouting seed dries out, it's dead.
As the seedling grows, surround it with more mulch gradually, keeping the leaves clear of the mulch, and leaving a ring of relatively clear soil right around the stem(s) if you can. Some plants are very sensitive to too much moisture right against their stems, so try to leave a little space there for ventilation. The airier your mulch (straw) the less this is necessary as the stems get older and stronger.
A carrot bed of scattered seeds is an exception here. I simply refuse to leave a little ring of space (using tweezers) around each plant. Live or Die, is my motto.
Now here is where I'm going to depart from Paul's info of warm soil.
I always read of warm soil. "Do not mulch around your tomatoes or it will shade the soil too much"... "Keep the soil clear around your heat-loving plants, esp in cooler climates"...
I did that and still my tomatoes didn't do well here in WA.
So I tossed that theory into the compost pile, planted my tomatoes, and mulched them gradually to about 6" deep with straw.
TOMATOES! BIG, RIPE, JUICY, SWEET/ACID TOMATOES! I was eating tomatoes off the vine, giving tomatoes to neighbors, relatives, delivering to friends, donating to the
local food bank.
So, I said to myself: "SC%$W THAT WARM SOIL THEORY!"
Now I mulch. And now I have a theory of my own.
As long as the plant gets enough heat on it's leaves, I think that's enough. And I think what was stressing my unmulched tomatoes was the feast/famine problem of too much water, and then not enough water, back and forth, all summer.
With mulch from birth (well, transplanting), I didn't have to water so much, so many of the soluble nutrients that were washing away with all the excessive watering were still available to the plants. (I was wasting water)
With mulch, the soil was protected from the sun, the wind, and competition from weeds. And the plants loved it.
Now, how do you think the tomatoes would feel about being mulched, say, in Texas summers?
Sue