The words do you take any discounts should become part of your vocabulary and before you hand disney any money. Never assume they do not take it or the list on the website or book was accurate. don't forget not everyone on property is a disney location to eat and some of them take other discounts or have their own loyalty. Some of those programs you can earn when you eat/buy but not always use it (starbucks for example). sometimes you can combine them or again earn loyalty points but not get the discount + the tiw/ap discount.
Pay attention to gift shops, if your going to get that bottle of water or soft drink just walk into the closest gift shop where you get a discount and get it from the fridge instead of the street vendor who doesn't take it.
If your looking at the pricy things around disney, ask before you decide to buy it because some of them, especially in epcot aren't actually disney stores and don't take it.