I don't understand how people manipulate the gun with the QD at the castle nut. That's not to say that there aren't plenty of people who can do it, I just always ended up having my sling getting in the way of manipulation there. I eventually ended up putting the attachment point on the strong side of the stock, like Alamo said.

The forward position is a personal life decision. You will get more weapon control with it all the way forward, but reduced ability to maneuver the rifle without loosening the sling (or just putting it around your neck). You can get better maneuverability with it at the back, but the sling isn't really doing anything for you for stability, if that matters. Depending on body type and length of the rail, putting it in the "middle" of the rail may interfere with support hand position (or it may not).

My personal setup depends on the rail. If it's <= 10", I usually have the connection a couple of sections forward of the barrel nut. If it's > 10", then I like to have two QD spots (or the equivalent). I normally run the gun with the attachment on the rear point, but if I need to make a longer shot with out support, I'll move the sling to the forward attachment point and use the sling as a stabilizer through tension.

For range toys, then I don't really care, and I just have one attachment point at the rear-ish.