It depends. Sometimes a little LITTLE bit of pain is actually enjoyable. I like when a girl sort of slurps up one or even both of my balls in her mouth, some guys will like that some won't. Licking the scrotum(ballsack lol) is also nice. The scrotum got lines on it almost like it is a burlap sack maybe trace along them with your tongue. Something the girl I lost my virginity did that I will never forget. We were having sex and I was doing her from behind and she reached back and cupped my balls in her hands. There was something very sort of romantic about it. All I can say is me and I think most guys like for a girl to pays some attention to our balls and not ignore them or be grossed out by them. They do look weird even we think so.
I wouldn't want a girl to be very rough with my balls, because it is pretty easy to hurt them. I do like having them licked and sucked, but they are very sensitive, so it really doesn't take a lot of pressure for it to feel great.
My balls are very large, and I think it would be difficult for a girl to get one in her mouth. But if she could, I would like her to suck hard, and chew on it, then pop it out hard. I squeeze my balls hard, it feels good. So yes, rough is good. Don't listen to these pussies who are 'too sensitive'. They're just little boys. Try a real man, like me, you can chew my balls off any time honey.
I say let him know that he can say when you are being to rough. They are sensitive but not to the point of where everything hurts it. Treat its sensitivity like eggs they feel like they can break and enough pressure will make it a nightmare. Its safe to say that you can ask him to tell you when it would be a good place to slow down or don't go any further. Sucking is nice but can hurt if to hard but I honestly can't see how any licking would hurt. Sorry if sounded perverted =/