It depends on a lot of things. It depends on whether your dates are included, it depends on whether your resorts are included, it depends on whether your room type is included, etc.
For example, let's say you have a WL reservation from 12/1-12/4 and a BC reservation from 12/4-12/8. The promotion might start on 12/1 with no blackout dates and include all resorts (best case scenario). The promotion might start on 12/8. The promotion might start on 12/3, but not include BC. The promotion might start on 12/1, but not include WL. The promotion could give higher discounts to Epcot resorts than other resorts, or vice versa. The promotion could start on 12/1 and end on 12/15, but the 3rd through the 7th is a blackout.
Since the promotions are based on the current booked occupancy of the resort, there are a lot of variables that come into play. There have been a couple times when my resort choice was more important than saving a little money, so I didn't book the discount. There have been a few times where I did change resorts or room types to save money. Most of the time I've been able to get my first choice of resort at the discounted rate.
Probably not the answer you were looking for, but that's been my experience.
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