High heels do WONDERS for your posture!
The heel is actually used by many women to force them to stand taller, straighter, and with their shoulders back. They also shape the calves nicely.
Having said that, I think what you meant to ask was whether or not high heels are bad for the body in general, and the answer to this is yes.
Your knee joints suffer when you wear heels and they're not great for your back either.
In the long run, I suppose this might hurt your posture, but not immediately.