After being a CRC member for years before moving to a PC membership, the tipping took me a while to get used to. However, it's something I gladly do now. In the CRC, sometimes drinks are mixed by the desk agent if the club is small and they are offended if a tip is offered. Even in the larger ones, the people behind the bar generally are not "real" bartenders, but rather "drink pourers." In the PCs I visit, the bartenders are excellent and mix great drinks.
I usually tip a buck per drink and the bartenders seem happy with that. Sometimes if I am there several hours, I will add a few more dollars to the tip jar before I leave. If I happen to be traveling during the Christmas holidays and the bartender is one I know, I will usually leave a ten or twenty on the way out. There are only two or three that fall into this category. The one or two times I have run across a surly bartender, I tip nothing. I just say thanks and wish them a good day.