Hi Snort,
This is an issue I have dealt with in pistons, and to a limited degree in turbines.
In the pistons, Cessna 310 in particular, there was a flight manual stated "zero thrust" power setting to use during training. As I recall form all those years ago, it was 1250 RPM and full fine (though I am subject to weak memory on the exact value). The rationale for this is that the prop was to be left at full fine in case power was needed quickly. With the prop in full fine, it would create drag at idle power which would exceed that of a feathered prop, because the air was driving the engine. By setting the power to 1250 RPM the engine would drive the prop just enough to balance off the force of the air driving the engine, and the net effect was equal to zero thrust. We did actually shut down piston engines and feather them at altitude a few times, but they are not so happy to restart, and let alone the obvious safety shortcoming of this, I think it's just un-necessarily hard on the engine.
In turbines it has been my experience that the engine was left running at idle, and the prop was feathered. during recent flight testing in a turbine powered DC-3, I enquired of a zero thrust power setting, and the training pilot with whom I was flying told me that they just feathered it and left it running, which we did. This was also my experience with Twin Otters.
That said though, I am a fan of establishing a zero thrust power setting, a touch above idle, and using that when close to the ground, so it is not necessary to wait for the prop to come back into fine pitch if you suddenly need the power. It is a pilot instinct to advance the power when it's needed, it's not quite the same instict to unfeather a prop, wait for it, and then add power. I think that any organization who regularly trains should determine for them selves a suitable training zero thrust setting, and use it by company policy. It does not need to be right on to be effective and safe, anywhere close will result in the desired training effect.
Pilot DAR