Centaurus,
You have accidently hit the nail on the head with the term "run down time".
The turbocharger impellor/compressor (turbine) happily runs at around 30,000 rpm.
To do this obviously requires both lubrication, and the cooling that the bearings derive from the supply of lubrication.
The engine, while running, supplies the oil, cooled and pressurised, to do this.
If we were to shut down the engine prior to the turbine slowing down to a rate where it could survive without pressurised oil,
then the turbine could spin unlubricated at 30,000rpm, serious wear would result,
the bearings would bake any residual oil, and a seized turbine would be highly probable.
"Cooling down time" is normally accomplished by the 5 to 10 mins of approach and landing power.
However, that little turbine is stilling humming away at 30,000rpm.
So, after we reach engine idle of around 1,000rpm after touchdown, we need to wait for the little turbine to decelerate.
This normally takes about 3 minutes`to get down to about 3,000rpm turbine speed.
Then we can remove the oil supply by shutting down the engine.
The manufacturer speaks of "rundown" time, not the mythical "cool down" time.
So, what we do is start timing from when we get back to 1,000 rpm on the engine.
If we need to increase engine RPM to near or`above 1,500 rpm, we spin the little turbine up again, so we again need to start timing from when 1,000 engine rpm is stabilised.
Fortunately all of the "timed" and EGT methods produce the same result,
they ensure the turbine is spinning slow enough to handle the loss of lubrication oil that occurrs when the engine is stopped.
Large trucks often incorporate a pressurised oil accumulator to supply oil pressure for a period of time after shutdown.
So if we understand what we are really trying to accomplish,
i.e. avoiding removing the oil supply while the turbine is at high speed
and delaying removing oil pressure until the turbine has slowed to idle,
then we can understand it is smart to roll through to the end of the runway, then backtrack to the parking area.
The benefit of that is that you absorb the 3 minutes rundown time
and you can normally shut down on reaching your parking area,
and your passengers aren't sitting around wondering why you don't shut down and let them out.