Are you sure about the horizontal positioning? By some laws of physics and my basic understanding of it, while miner is in vertical position, all heatsinks have the same sag and gravity is pulling them downwards at angle of 90°. And that may not be perfect, but I think it is best to stay in that position since if you put the miner horizontally, half of heatsinks are in perfect position (gravity is pulling them towards chips and that would make them almost impossible to fall off), but half of them are in worst position as they are pulled downwards (away from chips) at angle of 180° and that would make them (by some raw and stupid calculation) 50% more likely to fall off than in vertical position.
I understand what you are talking about, you are referring to the chips that are on the other side of the hashing boards, the small ones with small heatsink. However, what I am doing is putting upwards the chips with the long heatsink which are the ones that creates most of the heat and thus are prone to fall off the board due to this high temperature. The ones on the opposite side have a small heatsink which tells me they develop less heat. I am testing this position on several miners and see what happens.