Helikill isn't a problem, it's pretty easy to dodge them. I rarely get killed because of helikill. It's not like driveby that you don't stand a chance because people use auto-aim to kill you. A helikill requires skill, therefore it's ok in my opinion. Also you should be able (in most cases) to hear the heli arrive so you can run, or try to shoot it.
In some servers it's forbidden, but that's pretty lame because it's not a way of cheating. You're pretty vulnerable in a heli if you get shot at, and most people fly away if they notice they can't kill you. So it's really up to you, practice some I would say.
