Pat,
I, too, say that you should prewash. Additionally, I would also serge the raw edges as there will be a lot of shredding. Not all flannels are the same and they will shrink differently. Some flannels actually turn into limp rags and if so, you might want to use a backing to stabilize.
And, keep an eye on your filter in both the washer and dryer. Flannel is notorious for "plugging" up your pipes and hoses. This is particularly true for the rag quilts. They are dirty as they shed and shed.
When I made a rag quilt, I prewashed at home, but the washing after clipping was done at a local laundry.
Suz