To my thinking the key to it is never let a stone sucking vortex even form from ground to prop as once it forms it will start picking up stones.
Mic you are correct and I don't know whether the reduction in prop damage was due to less rocks being sucked up or that they still were sucked up but hit with less force. Controlling revs on start up apart from being good for a cold engine is so you don't have that momentary rev excursion up to say 1,800 revs. It only takes one rock to ruin the owners day. Also you will find when the prop is set on full coarse the revs are governed to not much over 1,800 no matter how much throttle is applied.
An interesting test for people with time on their hands would be to spread flour or similar in front of the prop and observe where vortices form at the revs needed to start the aircraft rolling both at full fine and full coarse. Any takers?