I've used plenty of flannel without prewashing, nothing bad has ever happened. In a quilt shrinkage is determined by the whole package... top, backing, batting, amount of quilting all plays a role. Personally I wouldn't worry about it. When the quilt is all done ( quilted and bound) I would ( and always do) launder the quilt gently with cool water then tumble dry. They have always come out great. I mix fabrics too, flannels with regular cottons, some washed, some not- I've never had a problem with the finished project due to laundering