How to get a room upgrade in Vegas?

[Creole]

I'll be going to Vegas Aug 7, and will be asking my g/f to marry me while we're there (it's a surprise). Anyway, I've heard that at check-in if you just ask to be upgraded (depending on how booked they are) they may give you an upgrade on the room for free. Is there any truth to this, has anyone done it, and how would you ask for the upgrade? Tip the check-in person, use the engagement story, any other better way? We're staying at the TI.