Upvote:5
Even if I do agree that during October you won't find super long queues, it's still a good to book some tickets in advance, where "in advance" stands for "one day in advance". The places you mentioned in your post are among the most crowded places in Italy so spending couple of minutes online would save you 30 mins to 1h of queues.
I strongly suggest you to book Vatican Museums & Sistin Chapel at least (again, one day in advance would be perfectly fine)
Upvote:6
It is indeed possible to buy tickets in advance for some Museums. However, for St. Peter's Basilica in Rome, it is not possible. If there are queues, it is because of the nowadays inevitable security checks.
Some of the queuing classics are the Uffizi in Florence and the Vatican museums in Rome. For these places you could buy tickets in advance. However, I would not bother about that. October is quiet as compared to other monthes, and the queues won't be too long. The time you have to spend in queues is more than compensated by the gain in freedom and flexibility.
In Rome you can buy a Roma Pass. It gives you free entrance to several sites, as well as free public transport. Depending on your interests, you can save money by using this pass. The advantage of the Roma Pass is that you can bypass the queues at places like the Forum Romanum and the Colosseum.