Mitty wrote:

I've been taught to set the prop to high RPM on final, but I don't like this
very much because then, when I run my checklist for the first time on downwind,
I have to leave one item "open."

So what's wrong with doing it on downwind? (This in a Cherokee 6 or and Arrow)
It seems to work fine, does not disarrange the engine, etc. I suppose there
is some small noise increase, but hardly much.

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

You want to land with the prop forward so that it is ready in case of a go-around.
Pushing the prop lever forward on downwind won't hurt the airplane, but you will
create substantially more noise, which doesn't help GA at all in an era where more
and more people are pushing for airport restrictions and outright closures. So why
not be kind to the neighbors and delay full forward?

AOPA has a video called Flying Friendly; among the items it recommends is waiting
until final to push the prop level to highest RPM.