I would never not tip a bartender in the U.S. (Specifying because in the UK it's actually a faux pas to tip the barkeep in a pub). Especially as I often order mixed drinks so I'm asking for a bit more than just grab a bottle of beer and open it for me. And having tried to mix my own, dang it, you guys have all my respect. And if I've asked for a custom, I give a little more, because I know that's work (One barkeep told me honestly the worst part about being asked for a custom is they have to do work to figure out what to charge you for it. MAKING it is fun, figuring out what the charge should be per whatever table they have...isn't).