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.