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Catherine - Your itinerary is splendid. Favorite place in Lake como to stay is Menaggio. Favorite lake towns to tour are Varenna and Belaggio. I would take 1 day from Venice and add to Rome to spend more time there exploring. 3 days in Venice is sufficient. Don't take a day from Florence. Book ahead for the trains between cities to get good deals.

Since the trip to Venice is going to eat up a good part of a day, I'd be hesitant in taking a day away from it. But I agree that Rome could benefit from an extra day. I'd take that day from Florence. Your choice, depending on how badly you want to do both if those day trips.

Sounds great. Siena from Florence you take the bus, which goes to the center of town. Pisa separate, one hour by train.

You have 5N in Florence. If you love REnaissance painting, sculpture and architecture you may not even want to leave the city for a day trip. On the other hand, if that doesn't float your boat, you can see the highlights in a day. Many people take the train to Pisa in the morning, then continue to Lucca for the rest of the day before returning to Florence. Siena is great, take the bus there. Now you decide if you want to take a night or two from Florence and add it to Rome. There's always more to see and do in Rome, no matter how many times you go.

The Freccia trains, which you'll be taking from Venice to Florence to Rome and also for part of the trip from Lake Como to Venice, have variable fares. When tickets initially go on sale there are some bargain prices if you're willing to commit to a specific date and time in the form of a non-refundable/non-changeable ticket. If you wait till closer to your travel time to buy those tickets, you may pay a lot more because other travelers may have bought all the cheap tickets. The Seat61.com website explains how things work.

Thank you all for your input!! Much appreciated. My daughter happens to be in Italy for a study abroad now. She's going to Venice soon and she said she'd let me know if she thought 2 full days or 3 full days would feel like enough (since she will be going again with us and knows what we like to do). I do think an extra day in Rome would be good since we will most likely spend one day in Tivoli and that would take a day away from Rome itself. I'm thinking staying a bit longer in Florence will make the trip a bit more relaxing than packing up close to when we get there.
I'm anxious to get our tickets so it feels more real :)