Yes, this is very common. You just have to teach them not to do it just as you need to teach a puppy not to mouth you.

Best thing to do is make sure your hands are clean when you handle them (don't smell like food) and make a high-pitched 'eep' noise when they put their teeth on you. Other rats will 'eep' at them when they bite a little too hard so this lets them know it's not acceptable.

As a sign of affection they will also lick and nibble you, just gently redirect them if it bothers you. Handle them more frequently and they'll stop with the test-bites and the grooming-nibbles shouldn't bother you.

One other thing that might help if you are keeping them as pets, is to teach them the word 'treat'. Only use it if you are giving them a treat and never use it if you are just playing with them. They learn pretty quickly that without the 'treat' word, they shouldn't expect food and are a bit less likely to do a taste-test.

I've had rats as pets for more years than I care to admit and I've never had a true bite from one. My bp's have bitten me worse than any of my rats every have.

Good luck!