I always wash all my fabrics before I use them, but with flannel, I do wash it twice in hot water and dry it in the dryer. It is amazing how much it shrinks...lots more than quilting cottons. Also, do not use the selvedges, cut them off! I do longarm quilting for others and never knew anyone to use the selvedges in their piecing and backings until recently...the weave is so much tighter that it deflects the needle when I quilt it and I hate to see what happens to the quilt when it is washed because they didn't pre-wash the flannel! With all the pesticides sprayed on the fabrics in the warehouses, I wash it all as it comes into my house...I don't want pesticides on my hands or in my lungs. I would not use a 1/4" seams either because of the way the flannel is woven, it is looser than cotton and will fray so much - a flannel quilt I did has had some separation in the seams, creating holes, so make sure that you use a full 1/4" or a little more for seams and quilt over both sides of every seam to help keep it all together. The more quilting on a quilt, the better it will hold together with use.