My general experience, as someone who often kisses ass and so often has his nose pretty far in there (though I normally eschew rimming), I can say the vast majority of professional dommes I've seen are pretty damn clean. I agree with the sentiments above that the vast majority don't even allow rimming, so won't have done something like taken an enema, but at least for my sessions (where I specifically ask for ass kissing in my interest list), they are generally clean clean. Which, again as pointed out above, does not remotely mean that there's no risk of disease if you actually lick, just that I'm saying that I can usually detect no smell at all.

IME, there are dommes like Goddess Jaguar above, for whom a trusted and devoted slave might get some rare privileges most don't (hell, I'd say the general principle of "top tier slaves get more privileges" just applies generally). Just lucky to find them. I find there are lots of bodyrub girls who are decently skilled in bdsm and allow this, though again, they don't advertise that this is available.