Well... I guess it depends on her previous preferences, if she's prude as a nun, or if she's been open to experimenting before and don't mind having something that smarts up her rear. If not, I would advise against it. If you're both open minded, I think the best time to ask would probably be when you're in the midst of the action and you're both a bit fired up. Such things can be rather uncomfortable to discuss outside the bedroom. And being prepared to receive as well as give is often the way forward.