The farther forward you mount the sight, the less of the field you will obstruct with the sight body, but the less field you'll have in the window. This lets you keep an eye on the field better, since less is obscured.
Alternatively, the farther rearward you mount the sight, the more of the field it obscures, but the more field you'll see through the window.
How much that matters for YOU is subjective. For example, I have a FastFire on a couple of my shotguns - prefer the SpeedBead mount near my eye for this, as opposed to a receiver mount, because I can see more of the field and better track select moving targets in the air - all of the movement is viewed through the window. Alternatively, on my 3gun AR, I mounted the dot farther forward to let me keep a better eye on the field so I can better see subsequent targets in the string.