Go to a local quilt show and look at the backs of the quilts, and you will have your answer. Some match, some do not, some are even pieced and look like "a coat of many colors." The point is, there are many ways to do a back. I think it depends on where it is going. If it is for someone that would be picky about the back or someone that likes the feel of flannel for the back like my son, I know to always put flannel on the back of his quilts. I also let the color of thread determine the color of backing. "To each his own" Do what works for you unless you are doing it for someone who is paying for it.