If you for example always work on residential interiors, or products etc, then mm would be a good system unit.
But if you work on large exterior masterplans then you'll run into viewport issues when navigating your scene if system units are set to mm (and vice versa). So m in this case would be better.
If you work on both very large and very small scenes then somewhere in between is better, so I use cm.
Inches in theory would be better as it's even more 'in-between' mm & m, but I tried it a long time ago and it created some stupid problems so you should definitely avoid mixing imperial system units with metric scene units.
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