Here is a small list of stores and their coupon policies are linked to the name. Print these off and take them with you when shopping! Trust me, this will be a great piece of paper to have when you are disputing whether or not Walmart takes internet printables! (Yes, they do!)
Target (Thanks KCL!)
I am actually quite surprised that a lot of stores, like Walgreens, Target and Albertsons, don't have a coupon policy available for printing from their websites. I recommend taking any issues you have at the register to customer service. Sometimes all it takes is a manager to help sort out an issue.
The general rule is that you can use one manufacturer coupon and one store coupon per item. Certain restrictions do apply, and occasionally you have to buy more than one item. Most stores do not pay overage if a coupon's value is more than the cost of the item, however, the cashier should be able to adjust the coupon down.
Also, it is a good idea to have a cheap "filler" item on hand to purchase that will make your total more than zero. In my experience, cashiers and managers don't like to "give away" free products, even though the manufacturer will be giving them the value of the coupon back, plus $.08!
I hope this helps with any confusion you may have had and any future coupon issues that may arise!