Attempted scammers 3 times on our 5 day Rome trip.
1 - Licensed taxi from the airport queue charged 60 Euros to Pantheon hotel, instead of the maximum set rate 48 Euros. Claimed meter had a greater fare.
2 - Licensed taxi claimed 18 Euro surcharge for picking up from Vatican, also wanted to dump us off "within 1 min. walk" of hotel. Refused to get out until destination, and refused to pay anything higher than the meter.
3 - Licensed taxi pulled over a few blocks after picking up and claimed 18 Euro surcharge from Collossuem. We refused to pay and were kicked out of the taxi.
GOOD ADVICE: Verify with each driver that you will only be paying the meter rate today. When we started doing this, no problems afterwards.