I agree that your primary place to mount your light should be what works for you and where your thumb could reach the button without moving your grip much.
With my 9" midlength rails it is more flexible (more rail area) because I could place it either at 3, 6, or 9 o'clock, but when I'm using my VFG I mostly mount it on the 3 position, sometimes at 6 if I don't want to hit my light with the barricade and rarely I use the 9 position when I'm not using my VFG.
On my shorter 7" carbine rails there is less room so mostly I mount it at the 3 position with VFG and at 9 without VFG, not much room at the 6 position with or without a VFG so I don't use that position.
The best thing to do is practice the different positions, with your weapon in safe condition turn off the lights in your house and clear all your rooms using all the different positions until you find out which one you prefer.