I am always very frugal and occasionally skimp, but clearly it is better to cover your backside. What I have discovered is that if I have extra around the edges, once it's cut off it's great to use for binding on another quilt. So having a good 2" all around is actually less wasteful. Better to be safe than sorry. I have a Sweet Sixteen and rarely have much shifting. I would hate to have to add the large amounts needed for long arm setup, though. Fabric is just too expensive!