At one of the JFK clubs, there's actually a sign that gently suggests that tipping is appropriate. I can't remember the exact language.

I will also say that I recently ran into the "world's worst bartender" at that location. More recently, though, another bartender was more adequate.

You also can't really get any mixed drinks at the JFK club -- not sure if that's the case elsewhere. I'm in the habit of tipping for mixed drinks in airline lounges and not so much for just pouring a glass of wine or beer. That said, I leave a buck (if I have one) in the jar the first time I get a beer, but don't usually bother tipping for a second round. Given the level of service I usually receive (these bartenders couldn't possibly get a bartending job in the "real world), that dollar actually feels excessive!