I had a friend in high school, well, she was more of a bitch who like to kick guys in the nuts and was a real bitch. Now that I think of it, the only reason why I hung out with her is because she was my buddy's girlfriend (later wife, later cheating ex-wife) and a few of my other friends had known her since they were in elementary school. Anyhoo, she would sometimes walk by and pinch my nipple or try to knee me in the nuts, and I would reply by smacking her on the arm. I never punched or kicked her, but I did backhand her several times a week.
Other than that bitch, I would never think of doing it.