Do you need to be on a tour to climb to the top? If so, what are the least expensive/ least crowded options?
No, you don't need to be part of a tour. I can't recall what the cost was but, as with all popular sites, getting there early or late is the best way to minimise the crowds, that and avoiding the high season and weekends.
Just be aware that it is quite a climb so, if you or any travelling party has any health issues, you might want to check out the climb. There is the option to take an elevator up to about a third of the way, but you still have over 500 steps to climb. It also doesn't suit those with claustrophobia, because the stairway gets very confined indeed and the walls lean more and more to one side as it follows the curvature of the dome. This is hilarious for those of us who don't have a phobia but potentially distressing for anybody who dislikes cramped spaces or has balance problems.
All that aside, it is well worth the climb!
AND there are separate stairs for ascending and descending, which means that you don't have to squeeze past anyone, but also means that you cannot turn back if you change your mind.
If some in your party don't want to do the climb, they can wait on the flat roof facing the piazza. Last time I was there they had a small cafe where you could get a cup of coffee or a cold drink.
Be sure to visit the open air roof, where there is a tiny snack bar and rest rooms, as well.
We were 6 older people. We all took the elevator and walked around inside. 3 continued on climbing to the top, the others had a snack at the cafe and waited for us. Like the other posters said, they also have a rest room and a lovely gift shop. We thought they had the best prices for gold and everything is blessed by the Pope. We were up there for quite awhile, it was a beautiful, sunny day and the views are amazing.
We did this on my second trip to Rome. It is a long walk, but you get to see the dome up close. MORE important, from the roof, you have a wonderful view of the square and Rome.
The basilica's website is not helpful but a non-Vatican tourism site says this:
"Schedule
October – March: 7am - 6:30pm
April – September: 7am – 7pm
The dome opens at 8am and closes one hour before the Basilica.The entrance to the Basilica is free.
Climbing to the top of the Dome:
Lift to the terrace plus 320 steps: 8€.
Climbing 551 steps by foot: 6€."
Least crowded is first thing in the morning. Security line for St Peters' opens at 7, believe the ticket office for the tower opens at 8.
On the climb, there are narrow steps in parts and wide steps and landings in parts. You can rest on the way, preferably not in the narrow parts where others can't get past you. I got the sense that there is a limit to how many people are allowed at any time, so the line gets longer as the morning progresses.