My wife likes for me to warn her, because she'd rather take a facial - she won't swallow, but that more than makes up for it, at least for me it does.
Other than that, I can do whatever, in fact she gets more turned on if I get a little rough. Nothing serious like slapping (I couldn't do that anyway), but some light hair pulling, some dirty talk, grabbing her face or shoulders, stuff like that. I always tell her it was awesome (because it always is) and return the favor if I haven't done it beforehand.