Because of the weight of the hook (and of the lead barb, if present), ball-head jigs will hang tail-down under a bobber unless 1) the buoyancy of a plastic body on the hook counters the weight imbalance, or 2) the knot is shifted toward the rear of the hook eye to shift the balance point backward (which works if the imbalance is slight).

A problem with curing the imbalance with the second method is that the knot slides forward when you have a fish on, and then has to be repositioned before your next cast if you want the jig to hang level.

Some jigs are designed to hang level. I suppose that if one put a buoyant plastic body on them, they would hang tail-up.

I'll have to admit that while I try to make my jigs hang level when they will be fished stationary or very slowly, I do so only because I read that this is important and not because I have any real evidence that it sometimes matters. Do any of you have such evidence--for example, the only thing different between the presentation of the person catching crappies and the one who wasn't was that one was ensuring that the jig hung level?