This is something I'd like to know (because I'm a nerd like that). The bar is torsioned depending on which side is leaning, so the opposite side is resisting torsion. In theory, the bar should never move enough to contact the differential cover. The reason I say that is that when the limit of the sway bar is surpassed, that's usually when tires will break traction. It also makes me wonder if that hump in the center of the bar is so the bar can be mounted closer to the differential, or for ease of maintenance of the differential cover. The reason being, the center of the bar doesn't play much of a role in resisting body roll, especially in hollow bars. Usually, in hollow bars, the material tapers from thinner in the center to thicker on the lever arms.If I remember correctly, the orientation does matter when the rear suspension is at FULL flex. The bar may tap the pumpkin or something like that. I don't imagine it would be anything serious...just a "possible" rubbing issues.
Now, this is probably going to bug me. I might call Hellwig soon and ask.