In most cases, the the tip and taxes are included in the bill, so tipping is not necessary.  If you have a large group, or have excellent service, a 10-15 % would be appropriate.

Ettiquette:  If you are a guest in someone's home for dinner, try to finish all the food on your plate.  Croatians consider it impolite for a person to attend a dinner gathering and not eat.  Also, just a heads up, fish is traditionally served baked, but with the bones still in.  Rakia, a favorite drink of the South Slavs, is also served at the beginning of a meal and tastes similar to plum brandy.  It is served in a small glass similar to a shot glass, and it would be polite to accept the drink, as it is often home-made.