Tour-a-tech told me there is no right or wrong way to mount panniers. Generally, for 1 up riding, panniers are mounted level to the seat with TT racks.
Happy-trail will tell you that they mount theirs at an angle for 2 up riding.
Others will say that a angle mounted pannier is to clear heals when riding with the feet off the pegs.
I think it boils down to complete owner or manufacture preference. On my KLR, I mounted the front about 5" lower than the front.