Okay, if that is the general with your models, you could even choose a template with centimetres (x10) or metres (x1,000) and keep modelling "as if" you were doing it in millimetres (i.e. no need to recalculate the values every time you draw something).
At the end, when you are finished, you can scale the whole thing down and in the model info dialog, set the units to mm (it will "stick" for this particular model file only).
If you do any texturing with image materials, only do it with the correctly scaled down model (images get scaled up/down when inside groups/components).
