You can go to 5 or 6 decimal places I would think.
You can also set up in Imperial units and run metric code by having a G21 in the code if you wish.
In reality however if you think about it, the taig has screws which likely are no where as precise as the small rounding error you are talking about, I have class 5 precision ground ballscrews on my mill and they are something like 0.018mm/300mm.
Hood
Edit, Ger beat me too it LOL