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On my current truck I would usually let the truck idle until the EGT dropped below 300. Figured that was a good way to tell i felt turbo had cooled down.
The debate as to whether it is needed or not - maybe it is not needed, but I like the option. If you run the truck hard enough before shutting down the turbo could still be screaming away.
Small turbos spin between 100,000 - 200,000 RPM's and don't just stop spinning because the key is turned off.