Sorry Quilt Police but I never pre-wash fabrics and that includes FLANNEL! I buy from Thousandsofbolts and other online quilting fabric suppliers from over the pond and in over 200 quilts, or more I have never had a problem!
Have I been lucky I don't think so if you buy the right materials for a job they work and I use quilting flannel a lot for my own and family quilts and they get used, and laundered a lot.