If any store gives me more money than what the math works out to from what I gave to what the total was, I always speak up.
However, if I purchase something, and it rings up lower, I never mention it. Its only happened a few times, and none are ones I could be 100% sure it was an error as they were at Walmart/FS/BB etc where they games could be on sale and the register notes it.
However when I purchased a X360 Elite at FS it rang up as $217. I looked at my buddy, he looked at me, I looked at the cashier, got the reciept and ran like a bat out of hell out of there