How much should I tip a taxi or private transport driver when in Italy?
Zero euro.
When in Rome do as the Romans do (actually when in Italy, do as the Italians do), therefore just pay the fare.
Tipping a taxi driver in Italy is like tipping your dentist or your attorney in the US.
They are not employees of a taxi company. They are well rewarded entrepreneurs who own car and medallion (and who are the biggest tax dodgers in the country). Italian law prohibits licensing companies, only individuals can be issued medallions.
In Naples last April we were told by our Mondo Tours guide that tipping was not necessary; not guides, not waiters, and not drivers.