If you're planning a cruise in conjunction with a WDW trip, I'd stick with an OBB.
Here's the problem - with a WDW trip, you have to plan at least 6 months in advance, right? IGT/OGT/VGTs generally come out in the 75 day or less mark. By then all your reservations will be in place for WDW, and you may not have schedule wiggle room. And there's no guaranteeing
if there will be a *GT sale on the date you want, much less when, no way to know if that's even likely until the cruise isn't selling at 4-5 months out. (Particularly a December cruise is going to look like it has decent bookings now with people holding their dummies there until they release the 2014 dates.)
I'm going to try to stop looking at the price of my Dec 2012 cruise as soon as I'm paid in full, so I won't know if it the price dropped. But for us, booking that cruise at the last minute isn't really an option since we have
DVC reservations on either side of the cruise at WDW, and its part of a vacation plan that is about more than the cruise. *GT rates are really only if you can not plan and just hop on last minute (I have one of these thrown together for 60 days from now since the rates were too good to pass up.)