Funny, I usually don't give any thought to this either but 3 years ago when we stayed at MGM for New Year's, they were insistent on checking. They were not allowing anyone into the hotel after a certain hour who did not have a bracelet on and these were only given to registered guests!

Of course, we had 4 to a room but only paying for 2 so we had to pony up and let them know. First time I had ever seen that. Stuck so well in my head that when we went back again this year for NYE and stayed at Monte Carlo we stupidly informed them that we had 4 in each room! They were checking room keys on 12/31 but we totally could have gotten away with it.

As others have said, the worst that can happen is you get caught, say you didn't know and pay the extra fee (it was 25/pp per night)!