I'm sure if you're really a magical princess, you won't ever need police. You can just set your foes on fire or teleport them away!
But seriously, getting back to reality, police are very important. A lot of them are well trained and really believe in their duty, but they have odd quotas and guidelines written by other people to satisfy. My older brother had trouble with cops for a few years because of one small mistake made a while ago; however, the problem there was legislation, not the police themselves. On the other hand, f it weren't for police, half my family would probably be dead by now, my grandpa would have never had a job, and I would still be putting up with a crazy stalker from junior high.
Most often, people only have problems with cops when they're caught speeding or drinking. If you don't want to get in trouble, then don't break the law. Simple as that.