I never trim a customer quilt, even if there is a massive amount of backer or batting left when finished. I do trim the bottom edge of batting if I need to roll back to do more quilting, since otherwise that extra can bunch up when reversed and cause problems---but I trim so two inches of batting extends beyond the edge. You never know what the customers plan is. She may want to fold the backer to the front for binding. She may want a binding wider than 1/4". If you've used double batting or something puffy like wool, enclosing the edge may take a wider binding to get a consistent width. I've never had anyone ask for trimming and never offered the service. Too many ways it can go wrong, especially if the quilt isn't square.