I brought the ticket online as there was a groupon deal, make sure you give enough time after the purchase as it takes almost a day to reflect in their database. Then you need to book a slot with them either in person or via phone, as they claim that if the purchase is made online, they have very limited slots.
They seat for the start of the tour at 15 minutes before the scheduled start time, but until sold out. On busy holiday weekends the tours can sell out during popular times, so it's best to reserve ahead of time or arrive early. For other less busy days, it is likely you can get a ticket right up until the point of the tour beginning, easily.
Our group of 3 just went to the counter just bought our tickets and went on the next 2pm tour. That was on a tuesday though so I don't know if they're fuller in the weekends, you might want to call ahead in that case.
I've only done walk-in and purchase, but I've not had an issue with having to wait for another session. I showed up about 15 minutes before the tour was scheduled to start both times.
Three of us just showed up, purchased tickets and got on the next available tour. Was very good..... Well worth it
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