In Copenhagen, tipping is not a tradition. If you receive extraordinary good service, you are welcome to reward it with a tip, but it is not expected.
Service is normally included in the bill at restaurants, hotels and taxis. Thus it is optional whether to round up the bill or not. Waiters and waitresses, taxi drivers etc. earn higher wages than in other countries and are not dependent on tips in the same way.
Tipping is greatly appreciated, but not expected, and you should only do so if you feel you are getting exceptionally good service. If you do tip, 10 percent of the bill is sufficient.