If you don't have an extensive knowledge of Rome and it's famous sites, it is well worth taking the guided tours, especially a 'skip the line' tour. However, it is a good idea to not take the first tour offer, but to 'shop around', there are quite a few 'tour touts' for the 'skip the line tours'  from organisations at St Peter's Square for the Vatican. They can keep tourists hanging around for long periods which is a waste of time. The tours enter through the museum entrance where there is usually no queue at all! Any member of the public can enter the Vatican this way and have a tour using the museum's recorded personal stereo for less than the 40 euros average price of a Vatican guided tour. St Peter's Basilica is included. The advantage of not pre-booking is that you are not tied down to a meeting time as  there is so much to see on the walk to the main sites, and having to get somewhere by a specific time may mean missing many interesting little sights.