When I did a flannel quilt, I heavily starched the fabric before cutting. I used a 1:1 solution of Sta-Flo liquid laundry starch and water, "painted" this on with a large wall painting brush until the flannel was saturated, tossed in dryer, then ironed with steam (which re-activates the dried starch). The flannel came out nice and stiff, and there was no distortion when cutting and sewing. (You might want to consider 1/2" seams for flannel.)
Starch is an organic product made from corn, potatoes, rice, etc. Best Press (and sizing) are created from chemicals. Sta-Flo is a cornstarch with an ingredient added to keep it from spoiling. Sizing (and Best Press) are lighter than the lightest starch I have ever used. Starch varies in stiffness depending on how it is diluted. The 1:1 above that I mentioned is about as stiff a starch as is possible to use and not have it become glue. (Elmer's washable school glue is actually very strong starch.)
Anyway, I highly recommend heavily starching the flannel before cutting. Starch also helps stabilize through the quilting process so you are unlikely to machine quilt in any tucks. Once the quilt is finished, all of the starch comes out in the first washing.