So years ago in days gone by, when turbos were high strung and attached to what would be considered basic engines (little if any engine management) you suffered a few things the most significant being Very high exhaust temperatures resulting in very high turbo temperatures.
Add to this basic lubrication setups and you needed some cold no low air to cool the unit down for 30 seconds or so whilst the bearings remained lubed (often with the waste gate open to minimise spindle speed).
Modern setups are much better, exhaust temperatures are significantly lower (thanks to modern engine control), oil lube systems much better and the turbos often have some oil reservoir capacity in them and the bearings are much better. As a result, its not needed, in fact the most highly strung engines often have water coolers in them and electric lube pumps.
So for a modern turbo petrol, I would say only needed if you have spanked it just before stopping (often taking it easy the last mile or so will be more than enough)
Throw in a diesel engine where the exhaust temperatures are even lower and the need totally goes away.
