Precision has nothing to do with the unit system. Or notation of fractions.

0,001m is as precise as 1mm

1/1000m is as precise as 1mm

In SI you don't even have prefixes, you use scientific notation with base units. You don't say neither 1mm nor 0,001m. You say 1x10^-3^. Which is exactly the same as the other magnitudes of this comment.

If you want precision in imperial, you could as easily say 0,00000000001 inch. It would be as precise as 0,000000000255 mm, or whatever the conversion is.