Many countries do not have tipping as part of their culture (eg. Netherlands); therefore. tipping is not really required but often appreciated. Other destinations, such as Hungary, it is considered rude not to tip. This can get confusing especially if you are travelling through a number of countries on one trip so it appears that 10% is a good rule of thumb. In North America, up that norm to at least 15%. But be very careful in Asia as it is often considered rude to tip and you may offend your server!
Also remember when tipping, it is imperative to have a good grasp on the exchange rate.
Below is a quick cheat sheet for tipping for many popular travel destinations.