i usually pick one and stay with it.. either the phase 1 lobby bar or the one outside before you take the stairs down to the beach. if you head to another bar - be sure to give them something.
Yeah the whole tipping thing was kinda weird for us too, We never really knew if we were over or under tipping... Also, if you're spending any time in the US tip there too, it's something like 15-20% there from memory (but check your receipts, some places will include it in the bill).
Dont think theres a set amount in mexico they say you should tip all it =s is the more you tip the better service you will get