I don't think this is a standard rule. I thought I posted it earlier, but just in case I didn't, I got lectured once by a waiter at a Denny's (lowest quality level diner-type restaurant in the U.S.) because when I was finished, I laid my cutlery parallel on my plate. He informed me self-importantly that the correct way to signal I was done was to CROSS the cutlery.

My husband also crosses his utensils to indicate he's done. I still don't, except on rare occasion.