To be honest: I never tip them, but I may offer them a drink. If they don't want it, that's it..
This wouldn't work in Portland, OR. Bartenders are not allowed to drink (alcohol) while on shift....OLCC (Oregon Liquor Control Commission) rules.
Incidentally, I always tip a buck per drink. I drink well scotch which runs between $3.50 and $5.00 per drink at most establishments, depending on whether it is happy hour or not. Essentially, I never tip less then 20%. This is the same as I tip for food. Unless I am running a tab, my tips are on a per drink basis. If I run a tab, it is between 20 and 25% of the tab total.
I have found that the better you tip, the better your drinks. Tip cheap and you get a skimpy drink. Tip well, and a shot is magically a double or better. If I am really getting crazy and want to finish off the evening with a couple of shots of Ouzo, the good tips really pay off. Note: of course I take a cab if I drink this much.