If we're going to angst over the "paper or plastic?" question (we're assuming, of course, that you left your reusable, green "I Am Not A Plastic Bag" bags at home "accidentally"), at least enormous multi-national corporations have made it so that we can minimize human interaction while making that crucial decision:
On Tuesday, IBM was granted U.S. Patent No. 7,407,089 for storing a preference for paper or plastic grocery bags on customer cards and displaying a picture of said preference after a card is scanned. The invention, Big Blue explains, eliminates the 'unnecessary inconvenience for both the customer and the cashier' that results when 'Paper or Plastic?' must be asked. The patent claims also cover affixing a cute sticker of a paper or plastic bag to a customer card to indicate packaging preferences.