I just removed the props and have the motors running at idle speed right now. I was going to let it run until it shuts off, which I think it does at 15%. But I'd love to hear if anyone has a better or another recommended proceedure.
After I've finished a flight, I sometimes just leave the props spinning at idle until the level (according to the IOSD) shows 7-8%. You can blip the throttle to bring the speed just below 'take-off' to run the battery down quicker.