Ground resonance...
I recall discussions of said practice (leaving the machine with rotors turning under power) during basic flight training. The general idea was to NOT leave a machine subject to gound resonance with rotors turning under power. Machines not subject to ground resonance was considered OK to leave. I guess it makes sense, and in my humble opinion it is worse to leave it with rotors spinning down with engine shut down because of the risk of blade drooping and sailing.
If my memory serves me correctly, machines with semi-rigid or rigid rotor heads can not get into a state of ground resonance, while fully articulated can (hence the oleo dampers on the 500, 300, enstroms and what have you, and the abscence of dampers on the Bo105 and the 206).
Point me in the general direction if I'm in over my head in this...
/p