Of course, that assumes that you *have* live rock! (Duh... just went back and reread your post and I see you say FOWLR
. Guess you do have live rock!) If those sponges are your only form of bacterial filtration, yes... rinse them out in the old saltwater.
The "nitrate factory" name gets tagged on to cannisters when they aren't cleaned out regularly. Even if you're not doing a weekly water change, you want to rinse/clean those sponges weekly to avoid finding out where the name comes from.