Having a dot sight mounted further forward tends to increase precision at longer ranges, as the fixed size of the dot becomes smaller relative to your target. With the very low weight of a dot sight, it seems unlikely that balance would be much of a concern. Aesthetically, I like dot sights further back. But for accuracy on smaller targets I prefer them quite far forward.
For one example in my own experience; I once managed to take a grey squirrel down at 35 yards using a PCP carbine with a Burris FastFire III mounted fairly far back. The 3MOA dot covered more than half of the squirrel. So I had to move the dot off my target several times to check exactly how it aligned, before risking taking the shot. I was successful, but in hindsight realized that I'd been very lucky. One really should always see the target when shooting at live game. Mounting a foot further forward would have reduced the dot to about 1/4 of the squirrel's size, making for a much easier job in aiming. But I've since switched to a 4x scope on that airgun. Much easier to shoot well that way.
