Essentially, the whole PCB world is imperial.
Its just that in the last 30 years or so metric values for imperial measurements have become the norm 
The old equipment that many of the longer established fab houses uses imperial, but they like to tell you that they use metric lol
The resolution of your data is important when you are working with fine pitch, 2.3 is just too coarse.
ISTR Gerber format is recommended by UCAMCO to be to 6 decimal places. A new Gerber format is currently in draft, changing RS274-X to X2.
I suspect this is to combat the new IPC format.
And its all going to be ignored by the old guys doing boards that will continue to use RS-274 and imperial. Yet they can still get their boards as cheap as those done using metric measurements.