When eating at a sit down restaurant, rounding up is appropriate and all the locals will give nothing more than this, however at restaurants in Prague's tourist district, where service is unfortunately worse, tips of about 10-15% are expected. Instead of encouraging poor service, only tip what you feel is appropriate at tourist restaurants or follow the locals and just round up.
Drinks
Mineral water, juices, and tea are popular and commonly consumed in the Czech Republic. Of course other drinks are also available, including coffee, soft drinks, and milk.
However, Czech Republic is known for their alcoholic beverages and beer is the national beverage, something that is very apparent after just an hour or two in the country. While the Czechs do have some imports, most beers are local from breweries that have given the Czech Republic a reputation as having some of the world's best beers. If you're looking for a more localized and less available alcoholic drink try becherovka, an herb-based drink, or absinth, which can have some bad side effects so be careful. If you want something more familiar, Czech Republic has all the popular international beers, wines, and hard liquors available as well.
Generally speaking, the tap water is safe to drink in the Czech Republic, but check with locals for any particular regional differences. Also, many people may have troubles adjusting to the local tap water, as it will most certainly be different from what your system is used to.