It really depends on the amount. In Belgium for instance there would be no tipping, but rounding up or adding one or two spare coins of change you still have on you in case the service was excellent.
It is like any job where people get a bonus because they have gone above and beyond.
Sorry for the delay, but yes, people have a tendency of rounding up if this is a possibility and it gets either charged to the person or put into a pot redistributed to everyone, depending on the local policy where everyone agrees beforehand.
This is normally on top of any bonuses for performance that the employees would get and their living wage which is guaranteed.
It is like any job where people get a bonus because they have gone above and beyond.