RE: Test stands for break in (not good)
Ahh, but Captin, most won't do it your way, which is the correct way. They'll let the thing run through a tank or a gallon of gas while they go have lunch or play a video game. I've got an engine I just picked up today that has almost zero compression. Over heated on a stand was the cause. The fuel mix and tuning was as good as great instrumentation could provide, but someone forgot to turn on the blower.
As for how much air gets through the prop when it's running, here's a nice test. Go find a beekeeper or one of their supply stores. Obtain a smoker. If you're careful a road flare might do almost as well. Pump a bunch of smoke in front of the prop and watch what happens to it and how it flows. The results are enlightening.
Anything within certain bounds and limits is generally not harmful, but to excess is where the problems arise.