I don't think there is a definitive answer, but here are my personal guidelines:
1. I would never aim at the head of the net person, i.e. deliberately go for a shot that would be long if he/she ducks and that is meant purely to intimidate.
2. Aiming at the body / midsection (with a shot that is intended to go in) is completely fair in competitive play (e.g. adult league, combo league). If the net person can't handle it, they should move back.
3. In casual play with lower-rated players, I would not do it - but would not mind if someone did it to me.
4. Mixed is a strange beast - one more reason I don't play mixed! I don't know what I would do. I think I would be ok aiming at a 4.5 woman at the net in 9.0 mixed (I'm a male 4.5). But I don't think I'd be comfortable hitting hard at a 3.5 woman at the net were I ever to play 8.0 mixed.