I know there are a lot of opinions going back and forth, so I'll give you my experience and you can decide from there.
I didn't wash any flannel the first quilt I made. It was a simple 4 patch and I was very nervous about it. I learned about an accurate 1/4" foot on that quilt. I washed it when it was tie tacked and it shrank so much half the seams cut right through the fabric. It was awful. Needed to be completely remade. There's a lady at my club who knows more about fabric than any other person I know and she swears flannel shrinks 2.5 to 3 times as much as regular quilting cotton.
I never used flannel again until this last queen size I made as a surprise for my DH for our anniversary. I washed it 4 times to make sure that it really shrank as much as it could. Ironing all the wrinkles out of it was a really nasty word. Backing 120" wide on my tiny tabletop board. Anyway, aside from the profanity when pressing it out, it worked great. The only complaint I have is that it was very, very hard to hand quilt. The needle went through fine, but my stitches are so small that it looks like the flannel swallows them up in spots.