I heard on one of the quilting shows, I think it might have been Fons and Porter, that if it is soft and flexible there is no need to pre-wash, but if it stiff and hard, you should pre-wash. If it is stiff, there is lots of dye and sizing, etc. and will shrink a lot. I've never used flannel except as a backing. I use it as backing on baby quilts. Never made a whole quilt out of it before.