CO2 has the approx. same weight like air - therefor it is not important whether you choose the wall or ceiling (as the manufacturer manual says). But place them close to the bedroom doors, so they will wake you up right away if anything happens...Good Luck

Thank you! I will definitely place one by the door in each bedroom. I'm just not sure where to put the great room one since it is open and has no doorway. Also, are they less obtrusive on the wall or ceiling? Maybe it's all the same. Probably whichever bay they fit best in .

Its better to install the smoke detector on the ceiling rather than placing it on the wall. To know more about smoke detector installation contact a professional electrician in your area.
http://www.lonlockwoodelectric.com/installations/detectors/

In addition don't install them close to HVAC outlets (if you have any), min distance should be 36".

My brother is a firefighter and I was told to put the CO detectors on the floor (by floor I mean something like in an outlet at the bottom of a wall). The CO will rise usually from the basement and you want it to hit the detector as soon as possible. We have CO detectors in outlets or attached to the wall at the top of the basement steps, on the floor in the kitchen, on the floor before you go up the stairs to the second floor and on the landing when you get to the second floor. Smoke detectors we have on the ceilings.