Jas, why don't you call the agents for Honeywell and Garrett turbochargers and ask them?
Kelly Aerospace Power Systems
1404 E. South Blvd.
Montgomery, AL 36116
(334) 286-8551
(334) 286-1992 Fax
For my area of interest in turbochargers (Rajay), they say this
Allowing a cooling down period for the turbo is a maintenance procedure that should be universally practiced. When taxiing in, allow the engine an additional two minutes of idle time before going to ICO. This simple practice will prevent any oil residue from coking in the hot turbine housing and will prolong the life of the turbo.
as
Reprinted with permission from Aircraft Maintenance Technology, October 1997.
I think I'll stick with that rather than the advice of the savants of an anonymous internet bulletin board.