It would be helpful to this addle-pated individual if du would output the same units as it accepts as SIZE inputs, so that one could more readily tell whether one was getting 1000-based or 1024-based units.
For additional clarity, it would help if for output the suffix were "B" as at present for 1000-based units and "iB" for 1024-based (and correspondingly it would be nice if "iB" suffixed units were accepted as input. As far as input goes, it's backwards-compatible; it's not for output if other programs are trying to parse the human-readable output, but maybe that's not a problem.