if you talk to the big boys, i;e 18 wheelers they'll tell ya if they are shutting the rig down they just let it idle for a couple mins while doing logbook or whatever, if you talk to the manufactures i;e CAT they'll tell you just let it spool down 30-45 seconds, this is to let the turbo slow before it loses oil pressure when you shut it down, they dont say anything about temps.
so what do ya do? when i shut mine down i just let it spool down, and thats only if i feel i have high EGT temps for one reason or another otherwise i drive it like a regular gasser, and i would point out, so do the majority of GM, ford and dodge pickup truck owners over many years with no problems to report. i think this buisiness of long idle cool off times is a thing of the past like a wives tale that may have had some merit back in the day but has since proven to not mean to much.