not all but most fruit type trees and some nut trees do better without grass. Some fruit trees, can even be killed by it, especially when the soil conditions are less than ideal for fruit trees.
Basically what happens is, the feeder
roots of the trees need oxygen, which they primarily get from the rain. The
perennial grass has a mat of roots that catch most of that
water and oxygen, depriving the tree.
Ideal would be to have the whole orchard planted to spike
root plants like burdock, yarrow, artichoke, nasturtium, comfrey as well as stuff that is only around for part of the year... spring bulbs like lilies, daffodils, garlic, ramps... there are a lot of other potentials. I suspect that thistle could be useful, to help deter pests or even livestock, as well as another good spike root. A lot of other "weeds" are good, plantain, dandalion, chicory, eccanacia and saint john's wart, dock? wild lettuce? cress? broccoli raab?
oh and of
course don't forget alfalfa and brambles, both of which are heavyweights in the beneficial insect department.
Also, where the summers are very hot, it is wise to plant fava beans under trees that provide some shade and some sun. By the time the fava beans flower, the trees have leaved out, which protect the flowers from burning. Remember though, if you are saving them for see this way, they also don't dry out too good, so you will have to pull them at the right stage and dry them elsewhere, lest they grow fungus.