I always use the red carpet line unless there's nobody in the economy line. I do, however, sometimes feel self conscious if there's a long queue in the economy line and there's nobody in the red carpet line when I walk up.
No - I don't bother letting someone know they're in the wrong line. It's a bit snobby to do so in my opinion.
I've never been in a situation where a GA has not acknowledged my presence in the red carpet line during general boarding, btw.