For me, tipping is always emotional blackmail. When you're expected to pay an indeterminate sum above the official price of the service, there's always an impli

For me, tipping is always emotional blackmail. When you're expected to pay an indeterminate sum above the official price of the service, there's always an implicit pressure to leave more rather than less for fear of looking stingy. Which is why the "unofficial, but official" percentage of tips always goes up. If anything, at a self service it's easier to tip 0, as no one is looking at you while you do it.