One of my boyfriend's boys chews on the bars incessantly, because we've accidentally reinforced his chewing by giving him treats right after he's chewed. I haven't figured out how to get him to stop chewing... but I'd advise that you're careful with treats and taking them out right after they chew, otherwise you'll accidentally train them that chewing is a GOOD thing.
Also, it seems like the rats that get the most time out of the cage are the biggest chewers. It's like the "Truman Show" effect - once they realize there's a bigger, more interesting world out there, they'll never be entirely happy in a cage. And even if you let them free-range in a whole room, once you let them see that there's a door to the outside, they'll chew the doorframe/carpet to get out there.
That's the problem with keeping such intelligent animals in a cage! It just means you need to keep them occupied by changing the cage around regularly, putting their food in something that makes them work to get it out, and providing a wheel for them.
Some ideas for keeping them occupied:
I like to hang non-pellet food above their heads to make them climb, jump, and generally work to get it. You can also wrap treats in newpaper in lots of layers, so they have to rip thru it to get the treats. Then they'll spend lots of time nesting with the paper. Also, I hang a lot of pieces of twine from the cage, especially by the door and corners, so that they get distracted with the twine and don't chew as much.
But most of all, I think the wheel is what keeps my rats from being unhappy. They run like maniacs, most of the night, and some have learned how to run simultaneously with others. I think every rat should have a wheel.