"Ugh my back, I really should book a massage but I'm always a little worried it'll tickle. Know what I mean?"
In this instance, having "some social grace" would be to recognize that whatever thrill you get from tickling is not shared by the general public, and that asking if she's ticklish is a normal and socially acceptable question which is in no way remotely comparable to asking if she likes her nipples pinched.Have some social grace and think about how the other person is going to feel when you ask them such a thing. Is it appropriate? Do they know you well enough to feel comfortable with you asking them such a personal question? You already know that you sexualize it - so would it be okay for you to ask her if she likes having her nipples pinched?