I often put flannel backs on my quilts even my King sized ones as this is what my family likes. I haven't had much trouble quilting flannel. I pin it well but do not spray baste. Some flannel does stretch more than others but if you have already washed it, you should be fine. I frequently use flannel sheets for my backing and they haven't given me difficulty either. I just need to make sure that the backing is well pinned and wrinkle free before beginning.