I never pre wash flannels or pre cuts..they ravel and it is a pain to iron. The fabric company's even recommend that you DO NOT PREWASH.
I love flannel quilts and I sleep with one every nite.
I have been quilting for over 40 yrs and also I am a longarmer. I make my flannel quilts, quilt them, bind the and then wash in cold water. The quilt all shrinks the same and I have never have had any issues.