Any time you use the card there's a (very small) risk. Quite a while ago I remember there was a waiter in New York (I think) that had a small magnetic reader hidden in his uniform. He just swiped the card as he walked away from the table to pay the bill to get the magnetic readout, and wrote down the rest in the back. After a lot of people reported fraud, they found the restaurant to be a common vendor between them. If I remember correctly, the restaurant owner let the police search the employee area and found the reader in an open locker.
After I bought my car last year, I found a reciept in it from the previous owner; it had fallen behind the glove compartment. Name, address, full credit card number, everything. It's downright irresponsible of any company to print that kind of information on a reciept. The customer knows all of that, they don't need it printed out for them.
