I always starch as I piece and have never had an issue with shrinking when I wash the final quilt. Works for me with pre-washed and non-pre-washed fabrics. The only thing you have to be careful about is that you press the block not "iron" you want to press it flat, don't move the iron around too much while it's wet or you will distort things and that's when issues start up.
Good luck!
Rachel