Here is the link to the official Wal-mart Coupon Policy. I recently had to e-mail the corporate office about two different cashiers not wanting to accept internet coupons. I got a call from my local store manager who apologized and stated that this was a "training issue". I now carry a copy of this policy with me to Wal-mart. I would suggest if you have problems, just ask the cashier to page a manager and have your coupon policy ready and visible.
I hope to research other stores and have those links in the future.