I have made two lap quilts and one baby quilt with flannel backing and have never washed any flannel beforehand. I quilted in the ditch, however, I did use a LOT of basting spray on the flannel and batting as well as the top, pinned it very well, used the walking foot and bigger stitches, and very carefully worked it seam by seam while checking the back often for any puckering. It took a while and washed beautifully. It think if you machine quilt without too large areas of no quilting, the flannel should work out well without washing first. It has worked for me.