I usually only starch if there are going to be off grain cuts, like triangles, trapezoids etc. I would starch as you go as it's best not to store fabric that has been starched. So you have a yard or two of fabric and you need 1/2 yard for your project, I would just starch the half yard. I wash pre cuts, but only starch if cuts will be off grain. so if i'm doing a jelly roll race, I probably wouldn't starch. If I'm sewing strips together, then going to cut the strips into triangles, I would starch the strata (so after sewing the strips together).
As far as starching blocks goes, if they are wonky & don't want to lay flat & square, I will use a bit of spray starch, but usually you don't need anything additional.
As in all quilting techniques, do whatever works for you. If you don't starch now, try it on your next project. If you like it, continue. You might find that it doesn't aid you in piecing, so you can omit this step.