I could stand to be corrected, but I've thought about this from a home studio POV over the last few months too.
When talking about a workspace where you've got the desk infront of you and you're staring at a blank wall (unlike an actual studio where you may have a window) then I would think that you may be limiting results by placing them vertically.
My train of thought would be to place them horizontally, one at the same height as the monitors and the other just above and preferably afew inches off the wall, since you don't really care about the reflections at your feet or well below your ear level.
Conversely, I would imagine that placing them vertically would still allow reflections on either side of them to interfere with the direct sound you're striving for.
