It's not mandatory practice to roll the throttle back, I would guess they chose to do it and it's not something that I do. However what I do make a practice of on preflight and post flight is to turn the blades backwards and listen very carefully to what's going on.
When mine failed it only had about 1500 hours on it. It's a 3000 hour component. It was sent off to Bell who determined that the failure was caused by the fact that it is in a 206 used for spraying and that it was not designed for the number of takeoffs and landings that we were subjecting it to.
I'll let others decide whether they agree with that or not. We had to buy the new one at full price.........