For what it’s worth, I can confirm from my own experience that they almost never check to see if the signature on the back of the card matches (my signature is messy and the space on the back of the card is rarely large enough to match what I sign on a receipt; Margie’s is an illegible squiggle that I think she chose in homage to her brother, the doctor), though they will ask for an ID if there is no signature there (or if the signature has blurred with time into a vague, bluish cloud).
(via J-Walk)