When you’re out at a restaurant or even at a business like a garage, it’s always nice to recognize good service when it’s received. HOWEVER, this is one of those points where there’s a significant difference in how we do things in the USA vs. how it’s done in Germany.
Forget about the 15-20% rule. If you do this, you will make the waiter/waitress very happy, but you’ll also run the risk of perhaps offending. How/why so? Because you’ll look like a “rich American” throwing their money around. One of the goals here is to help Americans fit in and shed the “ugly American” impressions.
If you’re ordering something like a cup of coffee that costs 2.50 Euros, paying your server 3 Euros is more than generous. Yes, I know that’s a 20% tip, but you can afford it, like most Germans. However, when you run up a tab of 50-100 Euros out at a nice restaurant, you won’t be leaving 10-20 Euros as a tip, you can probably easily throttle this back to 5-10 Euros and not be perceived as “stiffing” the service.