I prefer to buy the 96" wide roll, and then buy a few packages of King size for the few times I make a king quilt. I use to buy rolls at 120", and I'd always cut it the the smaller dimension. IE if I had a quilt 70*90 I'd cut the batting at 80" (70+10) and rotate it, but then I'd still have a chunk on the bottom I'd be cutting off (about 80*30) and I'd save them and put them together and use them up later.
That was a bit of a drag, and now that I buy narrower batting, I don't have near the scraps left over.