There are definately varying weights and qualities in Fannel, some when washed, well the life is washed out of them before you start!
As already said, I would wash a couple of times just to make sure it is well shrunk and see if you are happy with the results. Usually it will shrink more widthwise than lengthwise. Some wash up tighter and are ok, because of the soft finish on Flannel it is made with fluffy shorter threads and it is this widthwise thread which wears first, therefore I would cut all my backing using the lengthwise grain of the fabric, lengthwise on my quilt. Does that make sense? There are some I would not waste my time on for a quilt with so much work involved, if it is not durable.
Gal