In theory, you are supposed to show your AP at check in. However, I have used the AP rate more times than I can count and never been asked. Not one time. Not saying that other people aren't asked, but that's been my experience. I've also never been asked for the AP number when I call, just the appropriate code. I mean, really, who carries their AP around with them? When I book through the website, I do have to enter my AP number.