Well, it's not my personal experience, but I attended a workshop given by Harriet Hargrave. She brought an all-flannel quilt she had made to prove that even flannel does not need to be pre-washed, provided you quilt it moderately well (not lines 10" apart!). After quilting, batting controls the shrinkage. IOW, the fabric cannot shrink more than the batting. HH measured her quilt before and after washing, and it shrank exactly the amount of the batting (Hobbs 80/20, up to 3% shrinkage) -- not as much as one would expect flannel to shrink if washed and dried alone. I was able to handle this quilt; it was soft, beautiful, and not at all distorted.
Someone who wants to test this should quilt a piece of flannel, then wash the quilted piece along with a piece of the unquilted flannel. I think you will find that the flannel that is washed on its own will shrink a *lot* while the quilted flannel will shrink only as much as the batting allows.