Aesthetics aside, the lower and further forward the better for stability but gets in the way of the passenger and even worse is the catching and breaking your leg/foot. The higher and further back the worse stability wise but provides plenty of clearance for your legs to not get caught in them. It's a trade off. I went with narrow boxes rotated slightly and a little higher than most to ensure plenty of clearance. Here is a shot that shows my pannier placement somewhere south of Green river Utah.