I always book as soon as they open up reservations for the next year, and then add a discount later if one is offered.
I think it depends on how much you are relying on a discount. If you can't go without one, then it's not going to help to make your ressie early, in fact theoretically it could hurt the chances of a discount being offered, since more rooms have already been booked. (Realistically speaking, I know, what difference is ONE going to make, right??)But even if you already have a reservation when the discount is released, there is no guarantee that it will be available for the room and dates you have booked. So it doesn't really help to "save a spot".
But we always know that we are going regardless - I plan and budget based on rack rate, so if there is a discount that's just icing on the cake. And resorts do book up, sometimes quicker than you would think. A few weeks after we booked our April 2013 vacation, I realized I needed to change the dates to get cheaper airfare, and it was hard! Almost the entire month was booked solid unless I wanted to upgrade to a better view (which of course negated the effects of the cheaper airfare). I finally found something, but the dates weren't as ideal as I would have liked. Glad I didn't wait any longer to book or we might not have been going at all!
So I would say that if you think you can manage it without a promotion book now and know that you have your spot saved. If it's not a possibility anyway without it, then you might as well wait and see what's available when it's offered.