Oh, no! I have used flannel many times and only used a 1/4 inch seam! No one has complained - yet.
But, with flannel, I pre-wash and dry twice so that it shrinks before I make the quilt. I also use a denser quilting motif to keep it together. The fabrics/batting shift less when washing when more densely quilted.
That's my story and I'm sticking to it!