Ken, I read your post. I am a fairly new modeler, but when I mount servos, I always put the baseplate of the grommet in the top position. I figure when applying the screw pressure to to servo, the rubber grommet takes the vibrration from the engine and doe's reduce it to the servo and radio gear. To put it on the bottom, on a hard plywwod plate, to me, seems you just as well mount it directly to the plywood and there you are with vibration again.
I know the directions state, the brass washer on the downside, next to the plywood. You will not crush the brass plate in either direction because the screw is the same length either way. Just tighten it down snug, and that's it.
I just think that the rubber grommet provides a vibration reductuion tool, meaning it helps in reduceing engine vibration to the servos and radio, from the airframe itself. I just don't see the point of putting the brass washer on the down side next to the plywwod. If I am missing something here, inform me please!