The only time I've greased somebody was at the check in counter at a hotel in vegas. I slipped the guy a $20 and asked if there were any available upgrades. I felt like a fool because what if he said no and still kept the $20 lol, but he upgraded me to a suite. It was totally worth it.
If you work at a hotel or restaurant and you get slipped a couple $20s a night, over the course of a year, that's a pretty solid "bonus" and it's all under the table.
When it comes to Vegas, cameras are everywhere. Surely management knows that this sort of thing happens. Do they just not care or do they also take a cut?