I am not real sure what is correct for a diesel, but my thoughts are based off differences from 4 stroke engines.
To me the hardest thing on a diesel is starting when cold…diesel needs compression(friction/heat) to combust the fuel. More RPMs means more friction/heat and more power from the combustion cycle due to the higher heat(burn more efficiently and completely with higher heat)….when a diesel idles it cools. Diesel has its advantage when you get the higher heat to combust efficiently, which is not at idle. In general I think it’s better to run a diesel at higher rpms / load. However as I mentioned before I thinking starting is bad, but if you turn off for not too long the engine is still warmer than when cold and probably still lubricated…it’s much worse when cold and not lubricated….I can’t imagine that is good for the rings or much anything else. To me if at idle it’s not burning fuel efficiently so I wonder about what that does…I don’t think its ideal, but maybe it’s better than starting a lot. When the machine does a regen it uses higher rpms for a reason….nothing getting cleaned if idling. But I am most certainly not a diesel mechanic either so take with grain of salt…just my thoughts.
So based on i that rationalization(it could be wrong), for me if I am getting on/off a lot I let it run (2-3ish minutes per stop). If I am going to be away from the machine for a longer (10ish minutes or more) period I shut it off. So as an example when I am doing wood and loading wood in the bucket by hand to take up to porch and stack…I let it run when hand loading into bucket and when hand emptying to stack in the porch. If I am going inside, to take my boots off and drop a deuce, I am shutting it off. That’s just me / just my thoughts.