The thing is, contrary to popular belief, the timer isn't there to "cool" the turbo down, but to allow it to slow or stop spinning. Even when your car is "off" boost, you still have a certain amount of exhaust pressure causing the turbine to spin. About the only time the engine is not producing enough pressure to do this is at idle.
I fully agree with Ska in regards to driving lightly for the last few minutes - its a great idea for even NA cars, but the problem is, even if your turbo is only pulling 5ooorpm when you pull into the drive (as opposed to 15000-20000 rpm when on boost) do you really want to cut off its oil supply immediately?
IMO, it is cheap insurance against swallowing the innards of a turbo which will destroy your engine, no questions asked.