You do not need at reciept to exchange them at the parks. It would be impossible because many people recieve them as tips from guests. You do need the reciept to exchange them in TDS.
But if it worries you, just break all your $10.00 Disney dollars for something small in WDW. They give change back in regular money.
PS- I leave for WDW on thursday 9/06/01. I will post when i return and tell everyone about whether i could/couldn't exchange them.