JONA/John is the authority, so clearly you can just leave them around your trees.
However, if you just have a few trees, you have other choices that would not be very practical with acres of orchard. If it were me and I weren't going to do anything else with them, I would throw them in the compost pile with a bunch of "browns" (fall leaves, straw, shredded paper, etc) to become compost, which is a much more complete and beneficial soil amendment. (If you are not familiar with composting, check out the intro threads here: https://www.helpfulgardener.com/forum/viewforum.php?f=35
And of course, pressing them for juice / cider uses up a lot of fruit.