I lived in Germany for 3 years, albeit some years ago, and found that in restaurants they really don't expect you to tip as you would in the US. Rounding the bill up so that little change is necessary is usually appreciated as most of the wait staff collect payment at your table and it is usually in cash. Example: if the bill was 26.50 Euro, make it 30 Euro. This can sometime be more than 10% so even making it 27 or 28 Euro is OK. I believe part of the 18% that is added to every bill is somehow split among the wait staff but I could be wrong.