http://techdocs.shimano.com/media/te...9830750036.pdf
Check out the bottom right corner of this Service Instruction for a disc freehub. Unless I'm entirely wrong, rear disc wheels are recommended to be laced asymmetrically. It's probably something that doesn't really make that much of a difference, like where you break a Shimano chain in order for maximum strength, but like that, if there is a reason the engineers recommend doing it that way, I would like to know what it is.