AS350 engines, at least in the B & BA, went through a period where coking bearings was a concern with some engine oils. I was instructed to bring NR up to flight every 5 minutes of ground idle and run for half a minute or so. I've not heard of this issue in the last decade or so.
The collective will rise because the Starflex and controls are set/preloaded for a theoretical 50 knot cruise. Collective settings above or below this will be trying to return to this middle collective situation, the hydraulics maintain your commanded setting. The collective lock provides a positive lock to keep vibration and feed back from allowing this movement. Training is that once you have the collective full down, slide your hand up and engage the collective lock, unless you intend to fly away.