Sounds fine to me. The more you use detergent the more likely you are to get build-up so if you've used it once to get rid of manufacturer's residue, the rest of the washes don't need any (no poo after all) so you're really only washing for absorbency. But I did use detergent for my prewashes only because I just did it with the rest of my dirty nappies (once I had them) with quarter detergent - or with my white normal clothes with half detergent. (As didn't want to do a whole wash for just a few nappies KWIM)