I work in both millimetres and inches depending on what I am drawing and the materials I am designing for.

Always been frustrated by VC's system wide preferences in this regard, would prefer drawing by drawing preferences, or at least different preference environment templates.

On a Mac the somewhat awkward solution is to use separate user accounts for metric or inch (same approach could probably be used for any other 'preference' based changes.

Save into your 'Shared' folder from secondary account for easier access.

I know this is a bit awkward but is preferable to sorting problems caused by opening a metric file into the 'imperial' space with ensuing grid and conversion rounding issues when converting back.

I use VC 2D your mileage may vary with different versions.

In the past I experienced difficulties with drawings that were set to metric being opened while preferences were set to English, I am simply providing my work around to prevent those issues happening again.