I use flannel on the back of a lot of my kid's and baby quilts. Ok.. even adult quilts.. lol..
I haven't always pre-washed the flannel and it just made for a wrinklier quilt on top. I liked the look. It also shrunk the whole quilt about 3 inches, which was fine with me.
But I do think next time I will pre shrink.
And i use thin batting with it.