On Tipping:
Q: What is the etiquette for tipping a bartender at a reception with an
open bar?
A: For a reception with an “open bar,” you should not tip the bartender as he is being paid and tipped by the hosts of the event.
From Emily Post’s Etiquette Daily.
If it’s a busy event and you’re smart, you’ll tip a buck or two and ensure you get served rather fast next time instead of waiting behind a mass of people fighting for free drinks. A few bucks over the course of the night is a small price to pay for not having to spend more time than needed waiting for service.