Mine are 5-6" above the water.
The answer though lies in what type of reflector you are using, and the area you are trying to illuminate. The lower, the better (no lower than 5" though id say), but the smaller area you will light... but of course this depends on the height of the tank as well.
Id say try to get them as low as possible... 6" above the water is a good goal, as long as you cover the area you intend to. Rather than raise the light if the light doesnt spread enough, you can also use wider reflectors like the SLS lumenmax3, or the Lumenarc DE pendants.