That is USAirways policy and on all the flights I've been on they've been very good about it. Often first class is full with regular passengers (I know, you can't always tell who's an F/A, but normally the other F/A's on duty do recognize them and chat it up) and the few times it hasn't, I've only seen pilots sit down in first class after the doors have closed.

The last flight I was on had several seats open in first class and about a dozen flight attendants on it. None of them sat in the empty seats in first.

In fact, so far I've never for sure seen an F/A in first class on US. Just a few pilots. Of course pilots are easier to spot