I usually don't pre-wash fabrics and find they have enough body in them for accurate cutting for piecing. If you pre-wash, it's a good idea to starch all of the washed yardage before cutting.
What I do is mix up a 1:1 solution of Sta-Flo and water and "paint" the starch onto the yardage using a large wall painting brush. (This is a *lot* faster than spraying!) When the fabric is saturated, I toss it in the dryer, then iron with steam. I always use this technique on backing fabrics before piecing them, as the heavy starch helps prevent puckers when machine quilting. I also use this technique on any fabric that will be cut into bias strips, because the starch prevents the bias edges from stretching out of shape. This is also a great technique to use if you are piecing flannel, as it will keep the flannel pieces from stretching while you sew.
I use spray starch on the quilt top before layering. Again, this helps prevent puckers when machine quilting.
Spray starch can be used on wonky blocks to block them into submission. Sharon Schamber has some videos on Youtube showing how to do this.
Best Press and other spray sizing products do not have the same amount of stabilizing power as starch, and even starches come in different strengths. The 1:1 Sta-Flo/water solution is about as heavy as you can get with starch.