You can spray starch either way. I would probably spray starch the top while it is still damp, but after you get it blocked. I would spray starch the back after it is dry and you can turn it over.
I don't know if it makes a difference or not, but on the back side I would probably do layers of spray starch -- a light misting that is allowed to dry, followed by another light misting that is allowed to dry, etc. I think that helps it get absorbed into the fabric.
Let us know how it turns out!