you idle them down when moving stuff from the lawn , which of course you should have done before you started, moving garden furnature so you can mow underneath them or emptying the catcher
All good reasons to idle down an engine that does not have an "I AM THE STUPIDIST MORON ON THE PLANET" dead mans ignition that 99.9999% of the people tape to the bras in any case because it strains their fingers or gives them blisters.
It was somewhat of a rhetorical question and I know people do idle them down for the reasons you mentioned but is it really necessary? It's all personal preference of course but if I'm only taking 10 or 20 seconds to move something I just let it run full and keep as much cooling air flowing over that engine as possible.
When I bag I use a Snapper and I will either leave it a full throttle or shut it off. That set up on the mower is such that if you let the bag get a bit too full and idle the engine down to take the bag off, clippings want to fall into the discharge chute and at idle not all gets blown out. At full throttle it just blows it out. If I let the bag get way too full then I shut it off because I know I'm going to be having to pull some grass out of the chute and I don't want to be the cause of any more safety labels needing added to equipment.
I don't believe there is a wrong way. It's preference and what you've gotten used to doing. The only thing I believe is wrong is when people are mowing at less that full throttle.