I never trim the quilts unless the customer request me to do it. I give back all the fabric and excess batting. You paid for it and it is yours.
If I don't trim the quilt,the customer get everything in one piece. If she tells me that she is going to use the backing for binding I try really hard to position the top so she will have enough for the binding. I always ask for a min of 6" extra all the way around for the backing,this way I will not be short and the cust will have enough for her binding.
I have enough of my own fabric and batting and I don't need to feed the stash..lol