Not all fabrics shrink at the same rate.

Not even fleece. Ask me how I know!

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.

I don't wash my regular fabric, but I do prewash flannels. Regular fabrics usually have a 1-2% rate of shrinkage. Flannels can shrink 5-6% and more. That can cause a lot of problems in a quilt. If you really want to not prewash the flannels, then you would need to lots of quilting...which I usually don't do with flannels. I like minimal quilting to keep the quilt feeling soft, and not too stiff.

I don't prewash but I use a lot of water (steam) when I iron/press. I keep a spritz bottle next to my ironing board and spritz everything several times during the sewing process. I also figure that the quilting will help stabilize everything. I've never (personally) had a problem and I've been using flannel for many years. Many of my quilts are backed with flannel.