When I made a flannel quilt, I washed and dried the flannel in the dryer twice to make sure it had finished shrinking.
I starched the flannel too, before cutting. It made the flannel stiff so there were no problems with stretching and the pieces went together very accurately.
I think there is a recommendationsto use 1/2-inch seams for flannel in case it frays. I think I did that also.
Spray starch creates problems for me; my hand gets tired of spraying, I hate dealing with overspray (which I seem always to get), and it doesn't stiffen fabric to the degree I like. So, I developed the following method. Mix a 1:1 solution of liquid Sta-Flo laundry starch and water. Lay fabric on kitchen island. "Paint" starch mixture on with a large wall painting brush. Throw saturated fabric in dryer. Iron with steam. The resulting fabric (including fabric) is very stable so that even bias edges don't stretch. I use this method on background fabric for applique also, in lieu of stabilizers.