Roman cabbies do try to take tourists for a ride. I always take the train to Roma Termini (main train station), then take a cab or walk, depending on the distance (Rome is a carry-on trip for me). The train is an order of magnitude less expensive, much larger differential than in other cities.

However, cabbies at the train station have been known to go in circles with hapless tourists. A few words of Italian and ostensibly tracking the route taken on a map can help.