The water in spray starch relaxes the fibers so they can stretch.
I would say spray starch first if this is a piece where you will need to stretch it to meet the template marks. It will be easier to stretch the fabric if it is already wet.
Spray starch after if this is a piece that is too large for the template. Pin the dry piece to the template, spray to relax the fibers, then steam with the iron to get the fibers to shrink. (Drying wet fabric will tempt it to shrink.)
I don't think it makes a huge difference either way, just maybe a little easier.