Hi Angry Badger,
I do not recommend to use a metric grid size in the board editor since most parts are inch-based.
Using a metric grid and designing a board you will get a never ending list of "off grid" errors when you perform a DRC (design rule check). Additionally you'll have problems getting traces straight in to pads which then results in "angle" errors.
Using a metric grid in a schematic is not recommendable either since the symbols are all created on an inch based grid. Using a metric grid in the schematic editor will lead to unconnected nets, which you might not notice while drawing.
You can check if a net connects to a pin by using the "show" symbol represented by an eye on the top left of the screen. If the pin is highlighted there is a physical connection between net and pin. If the pin doesn't highlight you can be sure there is no connection.
The ERC (Electrical Rule Check) will not show you unconnected nets. It will put out error messages about unconnected input pins only!
You might use the equivalent values for several grid sizes like 1.27, 0.635, 0.3175. 0.15875 or even 0.079375mm and still get the nets connected, getting critical with smaller grids. You might to the same within the board editor.
If you save your work with a metric grid it should reload with the same grid setting.
Finally I recommend not to mess with the SCR files unless you know exactly what you are doing.
Regards
Boncuk