I am with Cruise Fanatic on this one. I keep my tickets and check them against the final statement. I have caught several discrepancies over the years, at least on CCL. Notably being charged for things I didn't buy. It's been better since they have to actually swipe the cards at the terminal now as opposed to just entering the number (they thought they had memorized).
They'll have the signatures electronically stored for any disputes, yet it can be a little late and a hassle if not caught before it is sent off to Visa Land.
Speaking of the grocery store, it is a weekly occurrence to catch errors on the ticket. Kroger's used to have the "scan-rite" guarantee. I got free toothpaste for a week once before they fixed the price.