I never trim the batting or backing before applying the binding. The extra batting makes an even surface for the feed dogs and presser foot. Also I like to leave a little extra batting beyond the seam so as I turn the binding to the back side to hand finish I can be sure the binding is full of batting. It is my understanding this is one thing judges look for. That is my goal to enter a quilt in a show someday. I have also heard fuller binding lasts linger.