Yup, I agree, it is so much easier to watch and catch mistakes BEFORE you leave the store. I try to watch them as they ring, and normally scan my receipt quickly when I get to the car before leaving the parking lot.
That being said, I normally know roughly how much my total should be as I add the numbers as I am adding things to my cart, never cease to amaze my hubby:)
Another thing to watch for is refunds that go back to your card. Just a couple of weeks ago, I went to Joann's during the big sale with the 20% coupon, and stocked up on pillow forms. As we were walking out of the store, I realized these were not the ones on sale. So back I go to return them and get the right kind ( It was a significant savings). She got my card back and put the refund amount back on it and I have a receipt that shows it! But I kept checking my card account online to see that money back and didn't for a whole week. I called the store and spoke to the manager, they got some info from the receipt and had to call their treasury who 'fixed" something. About three days later, the money did show up. I still don't get it, how could the receipt says the money were refunded to the card, show my card's last 4 digits and the money now show up! I wonder how many times that happen and gets missed. Buyer be aware!