Despite the fact that the standards of service in Poland have skyrocketed in the last decade, the average waiter/waitress in this country still only makes a paltry 8-12zł/hr (although there is a government move to set a minimum national wage of 13zł/hr), and a customary tip is still only 10% of the meal's total (though being a foreigner may make staff expectant of a bit more generosity). Those numbers look pretty low to us, so we strongly encourage you to reward good service when you feel it's deserved.
Finally, it is not common practice to add the tip to your credit card payment because the wait staff are then forced to pay tax on the gratuity; most likely you will not even have an opportunity to leave a tip on your card. For that reason, try to have some change handy so you can still leave a cash tip, or ask your server for change if you need to.
One new thing that we’ve noticed of late which we don’t like is the practice of adding a 10% service surcharge for groups of 10 or more - this is not always made clear in the menu. Why anyone would want to punish more customers beats us and anything that makes tipping obligatory puts us right off. We just split up into smaller groups as well….