It has been my experience over the past 6 months of using the DDE at Beach Club, Boardwalk, GF (multiple times), Poly (multiple times) and Contemporary, that I was not asked for a receipt.
There were occasions where I did not dine in the resort - once was for a Wishes Cruise from GF, and other occasions during the holidays to see the holiday decorations. I stated the reason for my visit on all visits, and only showed my DDE card to the valet attendent who parked my car, they noted it on the envelope with my keys. I have never had to show the card on my return to the valet stand to retrieve my car. I was also told by the security attendant at the gate when first entering the resort, that if I was parking valet, I did not require the 3 hour parking pass that they normally hand out.
I have also had them waive the valet charge when I had guests dining with me who arrived in a separate car. They other party were my guests at my DDs birthday party, and when I went out to pay their parking fee in advance of their departure, they valet attendant noted that it was DDE as well.
As I stated I can only speak to my experience.