Please, be sure to get an official TAXI

If you want the short Rules: Never get a taxi without identification number, especially when the drivers comes to you asking "TAXI???".
If you want to know more about, go on reading then...

In Rome, unlike other transport solutions, taxis are said to be expensive; unless you really need one (i.e. because you are lost, or you are carrying heavy luggage, or you might miss a plane, etc.), remember that from any corner of the city a bus/tramway stop, or a subway/raiway station, are always easily reachable.