The heat sink and fins at the bottom of the Mavic absorb most of the heat blown by the fan across the chips on the core board. No need to let it cool down since it stays cool by itself with the built-in cooling mechanisms.
Depending on the environmentals I may let mine cool for a minute. Good practice is to feel the heat sink on the bottom and motors directly after shut down. That will let you know if things are getting too warm.
Golden Rule: If the temp is uncomfortable for you - your electronics won't be happy either.
If i'm not mistaken, the act of it flying through the air helps to cool it down as well, so by letting it sit after flight might even let it heat up more.
Take for example when you do updates and it sits there on its own, the fan will kick in to cool it down. My logic says that when you land and the fan is already running, letting it sit wont cool it down more.
My procedure is as soon as I land, the drones motors and power are shut off. I will let it sit outside the case with power off after a flight just so that the heat sink on the bottom doesnt melt anything inside the case, especially on a really hot day.
I think it's more important however that after turning off the drone, to let it set out for a while to cool down before following it up and putting it into case. I always let it come to room temp before packing away.
Thats funny. I tried that very thing this past weekend. It didn't seem to be very effective though. What I do when Im flying from my house in the summer is after im done I bring it into the house and let it hover for a couple of minutes in the air conditioning. That really helps cool it off.