Warren,

The cool down of five minutes was and is intended to allow the turbocharger to cool down. Once you shut the engine off, there is no more oil passing through the bearings, and if the turbo is still extremely hot, the oil there will "coke" or burn. Once the turbo has cooled down if you were working it hard (your turbo boost gauge will tell you that) you can shut it down. If I have been driving relatively slow, like winding your way to a camp site when you turn it down does not matter because the turbo is cool.

This holds true for planes also, but like the bus, before landing the power has been pulled back for quite a while so it is unlikely the turbo is hot enough to be damaged.