If you're a couponer, you know it happens once in a while. You get a coupon for a free item and when you get to the register to redeem it, the cashier needs the price. You can't remember for sure and even if you can, most of the time, they want to see it on their register tape.
An easy solution? When you have a free item coupon, snap a picture of the price on the shelf and then you can just show it to the cashier. I've been doing this lately and the cashiers seem to appreciate it!