I made reservations about a month ago and they told me that they could take ressies through the end of the year. So, it's not like PS or a cruise reservation that can only be done 90 days ahead.
I don't think that the tours are exceedingly popular such that you really need to reserve them weeks or months in advance. Note that they WILL bill you for the tour when you make the reservation.
So, my advice would be this: If you have a specific date / time in mind and the money doesn't really concern you, book the tour as soon as you can. Then you don't have to worry about it.
On the other hand, if you're going to be at WDW for several days and don't have a strong preference for the day / time you actually take the tour, I think you could easily wait until a couple weeks before your trip to book.