My thought has always been the highter you mount them, the better it will be to light up the road and not blind oncoming traffic. If you mount them low (on the bottom of the crashbar) you will not only a lower profile projection of the road, you will get alot of drivers flashing their brights at you.
If the lights are mounted higher (even with the headlight) you will get the full benefit from them.
The reason I know this is because I had some small halogen driving lights (very bright) mounted low on my bike and ended up not getting much aded benefit from them. It also blinded on coming traffic (they flashed their brights at me alot) So I finally decided to go with a lightbar instead and it made all the difference in the world. Now I have one of them pointed straight ahead to light up the area where I need it and one shining over toward the shoulder of the road, which makes it alot better to see in the dark with just the low beam on. Just a thought.