So (1) is everyone taught to do it on final? (2) Why? (3) What's wrong
with
doing it on downwind?

1. Yes.
2. Rote procedure.
3. Noise.

I had this brought home to me during Mary's biennial. I was working in the
hangar while she was coming into (and going out of) the pattern, and, boy,
could you tell when she pushed the prop control forward on a fast downwind.
That Lycoming O-540 just growled.

Now, personally, I LIKE that sound -- but I understand there are those in
the flight path who don't. Since we have to live with those folks, it's a
good idea not to give them the ammo with which to shoot us down. Noise
abatement is serious stuff when your community is not 100% for having an
airport in the first place.

We now don't push the prop full forward until we're on final, when the
airspeed and MP is already so low that it makes no audible difference.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"