Cinque terre to rome

A home base to see what? Tuscany is a very large region larger than Massachusetts and Rhode Island combined so you need to specify what exactly you'd like to visit and whether you plan to rent a car or use public transport. For example both Siena and Pisa are in Tuscany but are rather far from each other. Florence would be the best base for both (and for much of Tuscany) especially if relying on public transport. Pisa can be visited in a couple of hours.

You can stop and visit Pisa for a few hours along your train ride to Siena. Buses run from Siena to some popular smaller towns, such as San Gimignano (fewer buses on Sundays), or guided van tours also depart from Siena.

I'm sorry I didn't mention that we are driving with a rental car to Rome and planning on visiting Pisa, Sienna and Tuscany ...This is our first time in Italy

With a car it is best not to be in Florence proper (or Siena). Both have severe traffic restrictions to non resident vehicles.
Find a hotel/villa/farmhouse in the countryside or a small town, preferably not far from the freeway that connects Florence to Siena. The Siena area is pretty central to most places people like to visit in Tuscany. Being sufficiently close to the freeway allows to quickly reach destinations. Tuscany is mountainous and highways other than freeways are curvy and slow.

Roberto is right on.

We had a rental car and saw Pisa and Siena from Montecatini Terme, which is also on a main rail line and easy to get to Florence in a short amount of time, as is Lucca on the way to Pisa. We also hit several other Tuscan towns like San Gimignano, Vinci, Volterra, and others.

We have also had a home base in San Gimignano and visited Pisa, and even CT, but as Roberto said, the roads are curvy and slow.

There are a million places to make a base in Tuscany and almost all of them will satisfy you.

You could stay for two-three nights in either Siena or Florence. I agree with Laura in that you can stop for a few hours in Pisa on your way from Cinque Terre to Siena. From Siena or Florence, you could take a day trip into the Tuscan countryside to visit a few wineries.