i have to completely disagree with you on that. idle is the most likely rpm for an imbalance to cause severe shake. from a physics point of view,
the amplitude (magnitude? i've been out of school for a while) of the vibration will decrease with an increase in frequency (rpm)

a few examples,
tie a long string to a doorknob, and hold a slight tension, pluck it and it will vibrate about an inch either way, and with 5 or 6 waves, tighten it, and you get less than 1/4" vibration, and 10-15 waves, and it's going faster.

harley davidsons. since the connecting rods for the two pistons share a comon throw, they might as well be a one cylinder engine. watch one at a stoplight. the front end will be vibrating atleast an inch each way, i saw a hotrod that lifted the front wheel a little with the vibrations....rev-up to go at the green light and it smooths out to a small, very fast vibration.

an imbalance that shows up at idle smooths out at increasing throttle, but still robs power from the prop.