I might tip the barkeep in USA lounges as I would in any bar. These people are contract employees and do not get handsomely compensated.
I leave a five dollar tip for the shower cleaners in USA lounges. Not only are they contract employees, they are minimally compensated and have a thankless job.
Beyond the USA, I pay attention to the national mores: Japan, Australia or New Zealand, tipping is out of order, for example.