At a bar it's most probably voluntary. There are some toilets where you have to pay after your visit but normally they put a sign on it, if it's mandatory. I only know it if there is a funfair or some kind of it. Like this: http://www.kem-zelte.de/wp-content/uploads/2015/04/1-IMAG017...

I'd say it's a good habit to always throw 50 Ct on the table - except if the person is as agressive/unfriendly as you describe. ;)