I've said this before: the way our legal system works, something is legal unless there's some law making it illegal. There's absolutely no evidence that topless is illegal on any beach in NC. Given that, I wouldn't expect any "known" topless area any more than I'd expect there to be any "known" areas where purple polka-dot bikinis are accepted: both are accepted anywhere and asking a law enforcement officer where a woman could go topless would have the same legal significance as asking where one could wear a purple polka-dot bikini. In the absense of a law to the contrary (and I have looked for one and asked here numerous times), those two activities are exactly the same in the eyes of the law.

Topless women are like barefoot drivers: many people assume it's illegal when, in fact, it's usually not. See this FAQ entry for a debunking of the "barefoot driving is illegal" myth. As to topless, there indeed are some places where it's illegal (St. John's County in Florida is perhaps the most famous), but it's in far fewer places than most people think.

If you're going to go topless on a beach that's patrolled by rangers or other law enforcement, it's indeed not a bad idea to let them know what you plan to do if you don't see others doing it, but if they say it's illegal, ask for a reference to the statue making it so because a lot of law enforcement officers are as naive in this area as the general public about what's legal and illegal. (E.g., women keep getting paid tens of thousands of dollars in NYC for falsely being arrested for being topless.)