All this is good insight and thoughts, as I have a Sig 556 with nothing but a quick release NcStar Bipod, vertical grip, and simple fixed power P4 NcStar scope. it has quad rails, and I also am in the market for something small, in a light category, not too concerned with a laser, but a light. Friends have the Ar's with lights at 3 o'clock but it seems to take away from the cover, or having to shoot from a doorway if the jam impedes the cover on that side.
I like the 6:00 position but a wiring arrangement seems subject to snags, etc,....
Not really wanting to accessorize my Sig but a good simple light seems like a valid one.