I've also done numerous baby quilts in flannel. Sometimes I pre-wash sometimes I don't. A quilt pre-washed doesn't shrink. One washed after quilting does but it usually shrinks then it looks fine because it's held in place.
What I was wondering about is routine washing of a queen size. Has anyone washed one? The weight could be a problem in both the washer and dryer. Line drying could bring my lines down. I really love flannel. That's the first place I go in a LQS. I prefer their quality over chain stores. Not cheap but in flannel you usually seem to get what you pay for.