Does it really matter? Would up on the roof where it will be under a panel be better than if it was down an the curved side? I know I want it right over the electric room and I can wire it later to the panels when I mount them.
I am thinking that as long as the seal is maintained, you can locate it most anywhere. I would think the only real exception is mounting it upside down so that the wire entry points would be exposed to the elements.