My kids eat a lot of dairy, but it does happen that we find that a bag of milk, cottage cheese, or yogurt will be sitting in our fridge past its expiration date. I have a few recipes ( e.g. pancakes, tuna casserole) that call for buttermilk or another type of fermented milk product. So instead of throwing out these sour products, I put them in the freezer to be defrosted when I have time to prepare the recipes. The end results taste delicious, and no one, except my pocket book, is the wiser.
Milk doesn't sour any more, it simply goes BAD. I wouldn't use it at all. The pasteurization etc. doesn't allow it to sour. I would try freezing it just before the date it expires. Saving money is great, but not if the savings makes you sick.