I am using up a bolt of 96" x 30' so I just cut it down to about 3-5 inches more around the size of the quilt. All those cut-offs get put along with others I've been saving up that I'll put together with batting tape to make up another batting. As I add each piece to the other one I keep tabs on what size it is so when I go looking for a batting I can check to see which one is closest to the size I need and then just add a little more to make up the size wanted. I hate waste.
I've been working on larger quilts so had to add to the sides all the time. So my last batting purchase I found a good sale on 122" x 25' bolt. True I'll still have to cut off the sides now but that's okay as they'll be longer pieces too.