Running Up a Cocktail Tab
When you're paying for an expensive, complicated cocktail with cash, throwing down a buck is generally totally cool. But, if you plan on running up a tab that requires your bartender to make 4+ complicated drinks, tip them 20%, because they've certainly earning it, and damn, you are having a fancy night.
Running Up a Beer Tab
On the other hand, running up a beer tab doesn’t merit 20%, because regardless of the price of the brew, pouring a beer is pouring a beer. Stick to the dollar per beer formula -- unless you're one of those dudes who's always asking for samples, in which case, tip heavier, because you're kind of a pain in the ass.