General protocol in Western Europe is that you tip those who deserve it, and not just for normal service ; if you become aware that the situation in a particular country is that tipping is a significant enough % of their income, then you tip ; also, if you have more income than most others, then you tip ; conversely, if you're impoverished, you rarely need to, but other points still apply regardless ; if you like the person, then you tip.
Mostly peculiar to Europe is the fact that it is quite acceptable to deliberately insult someone by tipping them inappropriately, various methods, though the most frequent is to leave some derisory amount of single-digit centimes, and direct a compliment to the person they are intended for (not necessarily your waiter -- it could be "compliments to the chef" or something ; though there's other methods, such as giving a tip larger than the meal price to some poor waiter unhappy to be working at such a trashy place). -- I'm sure similar exists in the US, but the norm over the pond that waiters should be tipped systematically does water it down somewhat.
I must admit that establishments that provide your change with smaller denomination coins in order to try and induce maximum tipping, induce me to leave zilch.