As an American living in Europe, I generally round up to the nearest euro, and usually add a euro if the bill is more than ten euro. This is not over the top, and most Germans do this as well.

It is important to note and beware, some airport restaurants and bars in the USA (the int'l terminal in Atlanta, for one) will automatically add a gratuity to the bill since they are used to dealing with Europeans who may not follow American tipping norms. Do NOT feel compelled to tip in the USA if a gratuity has already been added to your bill, unless you really believe that the server has done something to earn something above and beyond the tip their workplace has already added to your bill. Look closely at your bill. @:-)