Letting the engine idle for a bit before switching off

And there we go. Thought I'd be wrong. Damn! Ha.

Correct but a turbo that is idling will reduce velocity a lot faster than a turbo that has just come off load and has a high velocity of 80-100k rpm, as it still requires lubrication as it slows.
 
at work when we got new td5 land rovers
we were told to idle for 2mins the first start of the day, before setting off
30 secs idle, on next start
+idle for 30secs, before every shut down