If you breed a "true hairless" (the common hairless rat, not double rex) male to a true hairless carrier female, there aren't any of the same issues as breeding a true hairless female. Some people do still breed the hairless girls, but from what I've noticed, they always have smaller litters if they're successful at raising any babies (maybe half of them died and the breeders are just leaving that part out?). You would get just as many hairless if breeding a hairless to a female hairless carrier, but without any of the risks of loosing the litter.