I prefer a piece of flannel to no batting. Prewash first! Gives just enough to have something for the stitches to bite into. Also makes a color barrier which can be helpful, say if you have a dark back but pieces of white on the top.
I don't use flannel for backs very often, I have too many pets and the hair is an issue for me. Once it is prewashed, I don't have any real workmanship problems with it but not all flannel is equal! If you don't already have the backing, consider a high quality flannel sheet for the back instead of pieced yardage.