In the Iron Butt crowd, the prevailing knowledge is to put them as high as possible. Having them mounted higher on the bike gives you more aiming options. If they're low, about the only option is to have the beams aimed in a pretty flat orientation. If your lights are more of a flood beam, low might be ok. If they're more of a driving light with pencil-like beams, a low mounting position might not be as satisfactory.
The up side for having them low is that they create a triangle of light that, while maybe not being the best for YOUR vision, does make you more visible to OTHERS. When they're mounted close to the same height as the headlight, it looks like one big wide light to my eyes. The more points of light others see, the better they're able to judge your approach speed.
I have a set of PIAA 910s on my ST1100, and they hang from a bracket just below the mirrors. I have them aimed to cover about the same area as the low beam lights. I can use them at night without blinding on-coming traffic, and they make that coverage area like daylight! Using them during the day makes the bike look like it's got one massive headlight.
