I think it would be great if there was more documentation/clarity on how units are used. Typically when using state plane coordinates, coordinates are reported using US Surveyor's feet.
Even though this would be considered a standard: When exporting, using XML, the default is to use imperial feet. I'm not sure whether this would affect coordinates but I can see it affecting vector data built from coordinates.
It would also be nice to have clarity when doing coordinate transformations on what is happening- unit wise. Is the international conversion between meters and feet- or is the US survey foot being applied? This creates quite a headache when trying to backtrack.