I would like to wait until at least May to book our flights for a late September tour, just in case we are able to switch to an earlier tour for which we were wait listed. But some of the flights we are considering are already saying "only 3 seats left at this price." Do we really need to book now to get the best prices and the flights we want?
Unfortunately there is not a clean answer to your question. Generally the earlier your purchase the better the seat choice and flight options, obviously. When they post 3 seats left, at the moment that is true. Doesn't mean there might be three more seats at the same price a week later - or not. There is no logical to any of the airline pricing. Everyone wants the cheapest seat but it is like buying stock at the bottom price. Few people are that lucky. You need to set a price your are comfortable with and buy when it is close to that price. Most travel articles are reporting that prices will be higher this year and few seats.
I just purchased an open jaw flight to Munich and leaving Istanbul for September/October. I have been following the prices and know from past experience what is a good price or not. When I saw that the price was good, I bought my ticket.