Bar tender tips are really never based on a percentage, unless you are paying with a credit card for multiple drinks. Rather, bar tender tips are almost always a standard amount of cash, and IME varies by city. It used to be $1 a drink in most places. Now in major urban centers like NY and SF, that's not enough, and it needs to be $2 or $3 for a drink, or 20% if you are buying a whole round and paying with a card. Tipping with cash, in this case, actually saves money because you don't have to deal with percentages.