If someone is lacking on the hygiene side, then a tactful steer towards the bathroom is easy. I did once encounter a client for whom one shower wasn't enough, and it was clear he almost definitely didn't wash/change his clothes very often.
I recently had a guy not manage to wash himself very well in the shower (he was kind of a "if I don't have my fingers or penis inside you and my mouth glued to a nipple then I'm WASTING MY TIME HERE!" sort of guy, actually - and while I really don't mind an energetic booking, the ones where you just get frantically 'done over' by a man on a mission are not my favourite!) and all I had to say to get him to re-wash his willy was, "I'm really sorry but it smells a bit odd down here, do you mind if I do covered oral...?" He literally LEPT back into the shower almost before I'd finished my sentence. No hard feelings or bad mood or anything. Oral was still a bit tricky cos once your stomach's been turned it's impossible to un-turn, but meh. At least there were no arguments or anything. A polite question/instruction works wonders indeed.
(Ironically, with this guy, at the start of the booking when I'd left him to get showered etc and said my usual "see you in a couple of minutes!" - he replied, "Oh, I'll only need one, actually" to which I'd laughed uncertainly because that's a slightly terrifying joke and I wasn't even sure that he
was joking about not knowing how to was his own bits and pieces. And it turned out he definitely wasn't!)