I was thinking that maybe in the horizontal position, the metal plate might get warmer since the heat is rising and this might make dispersal of the repellent a little more effective. I don't have anything to base this statement on. Just seems like common sense to me. Mine also works in either position so maybe it is a mute point.
As an FYI to those unaware, You can use the Conair Cordless Curl butane canisters in place of the much more expensive Thermacell canisters. You do need an plastic adapter to make them work. Here is an example of what I quickly found on Ebay. I am sure there are others
http://cgi.ebay.com/20-Thermacell-Co...d=p3286.c0.m14