You really have a hard time finding the "rignt" answer and you may actually find several.
At ambient temperature CO is pretty close to the same density as the air so it doesn't pool on the ceiling like smoke.
Personally I think COs in the bedroom should be from mid level to low on the wall since that is where the air you breathe is but that is only my opinion.
I would say in any room they should remain accessible for regular maintenance, something that is not likely to happen with those "peak of the cathedral ceiling" smokes.
I would keep the CO as far from the fireplace as you can, just from the smoke contamination standpoint.
