All of our clients on private tours are interested in how and how much to tip in Croatia. For USA based clients tipping is a part of the culture, travelers from other countries, on the other hand, would not like to seem rude or go against common practices in Croatia.
Tips are a big source of income for waiters, guides, drivers so they definitely welcome it, but in Croatia tipping is not a must. Personally, when I go to a bar/restaurant/use taxi I tip when satisfied with service, a sort of an incentive for people working hard to do their jobs properly. My wife, on the other hand, likes to tip on every occasion, as she feels it would be rude not to tip. So you can imagine how our night out looks like when we get a mediocre service somewhere.
To sum things up, in Croatia tips are not obligatory, if you are satisfied with the service, you reward the person, if not you do not have to tip.
Restaurant tips are usually around 10% of the total amount paid for the meal.
Guides get from 20 USD to around 50 or 100 USD depending on the length of the tour.
Drivers, which spend more than a day with are usually tipped from around 50 USD per day, more if clients are satisfied.
Once again there is no rule, it is a matter of satisfaction with the service provided.