It's a loaded question that presents the word "rationalist" as some kind of straightjacket regulating minutae of your daily life.

Tipping is voluntary. If you want to do it, do it. If you don't, don't.

Couldn't you apply the same logic to anything, theism, lotteries? Strictly speaking following the law is also voluntary, it's just that there are generally unpleasant consequences if you get caught.