Tipping is a very personal thing.  Everyone is different.  Some people tip large, some small, and of course some not at all!  Many visitors generally tip around 10% or sometimes 20% of the bill.

Tips are very much welcomed.  In fact the staff, be it in a restaurant or a bar, may be surprised! They may try to return the money, thinking you have misunderstood the bill!

When you return to an establishment where you have previously tipped, you may be remembered and greeted with a smile and swifter service.  

In Slovenia a tip is not an obligation, but is appropriate for good service.

Info direct from Slovenian:

Tipping is not usual thing in Slovenia. Most of Slovenians do not Tip. If they do, they only roud-up the price, to pay round number (€9,90 payed as €10, nothing more). Only People who is travelling a lot and who appreciate the services do tip. So if you will expect from your waiter to return every cent, it won't be any strange to him.