Are you quilting on a frame? I quilt most of my quilts with backing only 2" larger all around. If it's not enough, I sew an additional 2 or 3 inches all around the backing and then mount the quilt. When I'm done, the extra fabric is cut off anyway.
If you're not quilting on a frame, I can't think of a reason why the same technique wouldn't work for you.
Good luck!