LOL you country bumpkins
No you dont need to leave it running at all. Thats only necessary on turbo cars when youve given them a good caning, the turbo will be running a lot hotter than the engine, so if you simply switch off, the oil flow and water flow to the turbo stops, cooking the bearings eventually if you do it too much. Hence why turbo drivers sometimes run turbo timers which let the engine run on for a certain amount of time even once youve locked up and gone in for a cup of tea.
NA engines dont need this at all, as there is no part of the engine that is that much hotter than any other - or at least nothing that matters if it stays hot for a while. Leaving the engine running will not cool the engine down any, only put more heat into it whihc will need to be lost out of the radiator.
16v and Williams have an anti percolation system which is basically an electric pump in the water hose, when the ECU detects that the engine is slightly too hot, say after a thrash in hot weather, it will run the anti-perc pump and fan for a little while longer after switch off, to cool the whole engine down.
