I always do mine AFTER a relaxer to repair the weakening done by the relaxer. It's usally a week or so afterwards depending on when I feel like doing a looooong washing session.
But after vigorously washing out the relaxer, I put DuoTex on my hair for about 4 minutes before I neutralize. I don't remember who from, but I got that from here on the board and won't go without doing it.