We also called and booked 'Behind the Seeds' in advance. Depending on when you are going to WDW, you may want to call ahead--we were told that these tours are especially popular during F&W Fest. When we called a few weeks ago to arrange for an early October tour, the choices of available times were already becoming limited (and I had to compromise, because the tour time I had originally wanted was already fully booked).
Also, remember when you book to mention if you are an
AAA member or Disney Visa Carholder, as both receive nice discounts on the prices for tours.