I wouldn't wash flannel precuts unless your project can tolerate a lot of shrinkage. Once you get the flannel pieced into a quilt top and *quilted* moderately, the quilting will prevent the flannel from shrinking too dramatically. Basically at that point the flannel cannot shrink more than the batting shrinks, because the quilting binds the three layers into one layer -- with the batting controlling the amount of shrinkage from that point on.