Tipping in Italy follows very different rules than in North America.
Tipping in restaurants and cafes is not common since most establishments automatically add a service charge (servizio) or a cover charge (coperto) to your bill. Leaving a euro or two on the table for particularly good service is absolutely acceptable but not required.
When paying for a cab (taxis), most Italian round up to the nearest euro: if the fare is 5, 34 euros they will give the driver 6 euros. While taxis service is very common in all of the Italian major cities, it may be more difficult to get around this way in the smaller towns. In all cases, always look for taxis that are either yellow or white, with a taxi sign and a meter.
Tipping tour guides and other service providers is always appreciated and appropriate if you feel that their service was more than satisfactory.