Yes, indeed I meant that 1 mil = 0.0254 mm, which is accurately represented by "reasonable" number of decimals, whereas other way around is not, 1 mm = ~39.37008 mil. Or it is exact if using fractions, 1 mm = 5000/127 mil
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I did not mean that would be good choice as a grid pitch, as it would not be even fine enough as demonstrated by the following example:
There is a similar problem for using imperial grid even with imperially dimensioned components if the pitch is small enough. Take a IC with 0.635 mm pitch (or 25 mils if you prefer) and even number of pins per row, now if you have the component origin at the geometrical center of the package (for the pick and place machine), then you would need a 0.3175 mm (12.5 mils) grid to route the pins using the grid. So one would fallback to decimals anyway at some point. PCB Matrix IPC-7351 PCB land pattern calculator defaults to millimeters, so "go figure".
Mentioned "optimum grid"-hassle is one of the reasons why I personally won't use a grid at all when routing, since the software I use will push away the neighborhood traces anyway according the design rules (unless I protect them, in which case it just won't let me route too close) when I route too near which would otherwise violate the rules, and traces will automatically snap into pads regardless which end I start the traces. It might be necessary to fiddle with grid with less advanced PCB software which can't do the aforementioned "plowing".
I believe the "professional" layout guys at my work do this similarly, or at least the grid is most usually completely off whenever I open the layout file for checking
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OTOH, I use something like 0.5 or 0.2 mm grid when placing the components, it is then easier to make them in nice rows. I never have had any problems doing that.
Regards,
Janne