We are considering car rental out of Rome and stopping in Assisi on way to lodging 30 minutes outside of Florence. Any suggestions re: viability of this option?
Sounds good. Start early so you'll have more time to visit St. Francis' hometown.
Spent 2 nights in Assisi last September. If you've never been there, you should take the time to go. Parking is just below the town, with stairs or moving walkways to take you up. Assisi is very walkable in a few hours. I'd suggest walking up to the Rocca Maggiore. The fortress/castle ruins at the top of town for a great view of the town and countryside. Make sure you allow yourself a good 3-5 hours to see Assisi. It's not large, but there are several churches you might want to visit and just relax and savor the feel of the town. If you get a chance, stop in at I Due Soli on Via A Fontini and say "Ciao" to Riccardo, and shop for some AMAZING handmade ceramics.
My wife and her daugther are doing the reverse. Taking a bus to Siena, renting a car for touring Tuscany and staying in 1) an agriturismo near Siena and in Assisi for a couple of nights and then turning the car in at Orvieto. Tour Orvieto and finally take the 1hr train into Rome. We've been to Assisi before and it's our favorite hilltown. To us visiting Assisi is a great idea. I suggest that you really need a good 4-6 hours to do this. With the steep hills in Assisi, I suggest that you take the Line C bus to the top and tour as you walk down the hills. The Basilica is at the lower end of the town. Great small shops and places to eat. Eat lots of gelato.