I've had this debate a few times before. It seems, though, that I tend to make friends with people, only to find out later that they tend bar. Their occupation has no bearing on my friendship with them, only that, at some point, this conversation will come up.
What is an appropriate tip for a beer ordered at the bar? I''ve had to listen to many a bartender complain about getting crappy tips from the beer drinkers. They order whatever beer is on special, and then only tip a quarter or fifty cents. I'm of the opinion that this is an acceptable tip. If you order, for example, a 75 cent coors light or a $1.50 bud light, the remaining quarter(s) you get for your change should be the tip you leave for the bartender. The bartender, for all the effort of getting a beer from the cooler and opening it, gets a 33% tip. For about 6 seconds worth of work, you got a percentage tip that any waiter only dreams about. They insist on being pissed at the customers for leaving small tips, when it's really the bar owner they should be upset with. He sets the prices, and fully intends for the drunk people to leave this size tip. That's why they don't make the prices come out to an even dollar, so there will be change left to tip with.
Some of my bartender friends are not my friends anymore after this conversation.