Fly into Naples or end trip in Naples (Sorrento)

It is typically easier to fly IN to Venice than to fly out of
I don’t believe you’ll find direct flights from/ to either Venice or Naples

If you have to fly out of Rome you can go to Naples/ Sorrento from Florence then back to Rome for final nights

Venice is a great place to get over jet lag, a decidedly “easier” place to be than Naples or Rome so I’d try hard to start in Venice

I would recommend that you read the reviews of the Naples Airport and factor that into your decision. It is avoided by many who travel including myself. It is difficult to transit through and the customs/Immigration area is a long hall with too many people crammed into a small space.

I didn't find either airport unusual, except maybe off-hour flights. You can always book a taxi boat in Venice if you're off hours, but it's hard to get to Sorrento at night.

Weather-wise, I'd prefer to start in the north and work my way south for a September trip.

I would plug in the flights and see what the actual results are--cost and schedule-wise.

I would fly into Venice. There are no direct options from California to Italy, but Venice has more options than Naples. Also flying out of Venice requires very early wake up as the airport takes time to reach from the historical center in the middle of the lagoon.
You should also consider leaving from Rome, which has the most flight options. In this case you would leave Rome as the last destination of your trip, with Sorrento right before it.
I would also do Florence after Siena. In sum this would be my sequence:
Fly to Venice
Venice
Siena
Florence
Sorrento
Rome
Fly out of Rome

acraven nailed it, i'm going in may starting with naples and ending in venice ;)

Thank you for your suggestions. I like Roberto's itinerary.