The short answer is, it depends on when you visit! It is not necessary to make a Pantheon reservation if you are planning a mid-week visit, as the Pantheon is now a church, and so entry is free on weekdays. However, during the weekend, it is necessary to make a Pantheon reservation in advance, which means you will need to book either a guided tour or an audio guide.
Due to the high volume of visitors, audio-guided Pantheon tickets and Pantheon guided tours come with an allotted time slot as standard, so if you book one of these options, your reservation is included in your tickets. It's recommended to book your audio guide or guided tour as far in advance as possible to avoid disappointment on the day.