Actually, I'd say if you're staying in the main towns in Tuscany, from Florence to Siena to Pisa, Lucca, Arezzo or any of the other main towns - the best way to move around is with the train. You avoid having to deal with limited traffic areas where only residents can drive in and not have to worry about parking which is generally difficult to find and expensive.
Having said that, if you want to explore the smaller towns, such as the hill towns in Chianti, a car is definitely useful because there is no train that connects them and buses are very limited.
Your itinerary so far includes Florence, Siena and Cinque Terre - all doable without a car. Have you been to Tuscany before? You would like us to recommend a base for a smaller village but what I'd recommend totally depends on whether you've been here before or not, since if it is your first trip I'd recommend different areas to stay in and explore.