A google or yahoo seach on "prague taxies" might answer your question.

A sample of one, from eveyones favorite, Rick Steves.

http://www.ricksteves.com/news/archive/0299taxi.htm
Prague's taxis, the most dishonest in Europe, are being tamed. While bandito cabbies still have meters that spin like pinwheels, the city has made great strides in civilizing these crooks. Though most guidebooks advise avoiding taxis, this is defeatist. I find Prague is a great taxi town and use them routinely. When you get the local rate, they're cheap. Here are some tips.
-------------------------------------------------------------

While cabs have a bad rap in a lot of places (NY, Buenos Aires, Bangkok, etc.), I've always heard about Prague as an extreme example. I agree with Steves that avoiding taxis altogether is defeatist, if you take some time to figure out what's what. (I was only in Prague for 14 hours, and walked everywhere, anyway, except to/from the train station/airport; if I had been there longer I would have taken cabs).