Another dedicated pre-washer of everything here. But even if not, I would still prewash the flannel just to tighten up the weave. It will be more stable to cut that way, with or without starch (I'm also a starch lover/user).
Overall, most of your quilt shrinkage will come after it is complete and it washed due to the batting, typically, having the highest shrinkage rate of the whole sandwich.
I wash on a permanent press cycle with warm water/soap. Dry on a permanent press cycle as well. Just reduces the wrinkling a bit more.