We are traveling to Rome for the second week in December. Do I need to book all my tickets for museums and attractions in advance or since this is the "low" season can we wait until we get there.
Go ahead with the hotel for sure as there is no low season in the capital of the Roman Catholic church.
But the most popular attractions may not be bookable 5 months in advance.
Tickets you should definitely book in advance:
Galleria Borghese: booking advance, timed-entry tickets is mandatory for this one.
Colosseum (these will be timed-entry)
Vatican Museums (these will also be timed-entry)
It is too early to book any of these for December with the exception of general-entry tickets to the Vatican Museums. I see that those are currently bookable for December; just not their tours.
Official ticketing sites:
https://www.tosc.it/en/artist/galleria-borghese/galleria-borghese-2253937/?affiliate=T2C
https://ticketing.colosseo.it/en/categorie/singoli-1-8-persone/
https://www.museivaticani.va/content/museivaticani/en.html