To keep the jig level I use the palomar knot and move the knot to the back of the eye; towards the hook point.
Many people use the loop knot but I've never seen it keep the jig level. The jig in the demo above has no tail material and a spinner on the head which makes the forward part of the head heavier. A plain jig head with tail or a plastic body won't hang level unless it's the bullet type head.
I tend to agree that the loop knot is not designed to keep it level. I think the loop knot is superior to the palomar knot in allowing the jig to move around within the knot giving it better action. So does better movement catch more fish than a simple level jig? More research is needed!