Note: Before you read the below and start telling me how you dont have to pay for it if someone steals your card: All the cards i have include a limited liability clause, but that is NOT related to the reciept or the back of the card, its a seperate service that covers the card period.
A friend of mine worked at a shell station here. At the end of the week they take all the reciepts and destroy them. Its in the same boat as signing the back of your card, it makes people feel warm and fuzzy about using it, but doesn't really do anything. Call your bank up sometime and ask them if you have to pay the bill if someone uses it and forges the slip. Or if you dont sign the back of the card and the walmart person takes the card anyway if you have to pay the bill.
Whenever i have to sign a slip i scribble the first letter of my first name and then some wavy lines and the first letter of my last name and some wavy lines.