They are optional...I think the only exception is if the menu says a service charge of 18% will be added to parties of 6 or more. I tip anywhere from 0% to 40%....it depends on the service and if it is our regular bar/pub (they get more because we are there weekly). However, in the big picture, don't lose sleep over a 20% tip on a beer before taking a cruise that might cost $1,000 per person, per day. The bartender is not making a living wage on his salary alone.