I would starch the flannel backing. Keeps it from distorting while you machine quilt, eliminating puckers. The starch comes out in the wash.
Heavier starching is better than light starching. For a backing, I mix Sta-Flo and water, "paint" it on the fabric to saturation, toss in dryer, and iron with steam. This is much easier than spray starching and provides a heavier starch that keeps the fabric from moving while machine quilting.