I love flannel for the back of quilts. I usually always use flannel. It is so soft and cuddly! I never prewash and have never had a problem. I like the look of the quilt when the fabric shrinks. The gals in my friendship quilt group do not prewash their flannel either. I agree with the others that without some type of batting there is no definition to the quilt stitches. I make charity blankets for foster kids and don't use batting - just flannel on the back, but I tie those instead of quilting. They do make a nice lightweight blankets.