Do you really have to ask? In line with TSA's utter lack of competence, as long as you buy a ticket within the 72 hour window before the flight, nothing happens if you don't give them that information.

I don't know what happens if you buy earlier without filling it in. Probably not much, either. And even if it did, all they could do is print something on your boarding pass (which a bad guy would of course just remove).