Yes, tipping isn't really considered optional over here. But there are certain times when you know it'll benefit you in a really crowded setting to slip your guy or gal a few extra bucks. We did the pre-pay deal for bottle service at City that I mentioned before and slipped our server (who was very outgoing, engaging and attentive) a $20 when he came to refresh our drinks (there were about eight of us together so only a few bucks a piece). He showed up 10 minutes later with two brand new bottles and said, "Sir, I'm so sorry that you never received your first bottle, here are two that are on the house." Then he kept showing up with some sort of tequila/margarita type thing that we kept taking shots of. Then they gave us wrist bands for an open bar after. As you can imagine we got really, really messed up.
At another Cancun bar, we got two beers and tipped the server the standard $1/round and he came over with his hand out and said "You guys are going to take care of me at some point, right?" No wonder the rest of the bar was empty.
In Cancun it's definitely expected there'll be some sort of tip - but in Vegas, for example, before you even have to sit down at a table you have to sign a piece of paper with a "service charge" on it, followed by some sort of handshake agreement with the host/hostess before you get served. Even as Americans we think it's unbelievably stupid, so I can't imagine how people from elsewhere feel forking over a pretty penny.