I usually do not prewash fabrics and find it unnecessary to starch them. They seem to come off the bolt with enough sizing/starch to be stable for cutting and piecing.
I heavily starch a fabric, whether it was prewashed or not, if I am planning to cut it on the bias, or if I am going to use it for a quilt backing when I intend to quilt on my domestic machine.
If I prewash fabric, then I always starch to restore stability to the fabric before cutting.
I use a heavy solution of starch on fabric that will be the background for machine applique, whether the fabric has been prewashed or not. Heavy starching eliminates the need for me to use a stabilizer underneath the background.
I spray starch tops during my final press before quilting.
For frame quilting, I use spray starch on the backing as I load the backing onto the rollers (which I learned from a Sharon Schamber video). The top is already spray starched.