Do not take a taxi unless you have called ahead by phone, have had a Czech call for you, or are travelling with someone who speaks Czech. The cab drivers will take you for a ride in more ways than one. But the prices they charge locals is much different. Definitely NEVER take a taxi that is waiting in front of a tourist area. They are the WORST. Also be careful about Airport Taxi drivers. They will rip you off big-time from the airport. You might not be able to help that on the way into Prague but on the way back to the airport, ask hotel or bar man that speaks Czech to order you a cab.
There has been many reports that when passengers has been arguing about price some taxi drivers use inappropriate aggressive behaviour. Agree on price before you get in to taxi.