Quote - How accurate can the grid plane be made?  Can the visual grid be set to respect accuracy down to a tenth of millimeter as well?

What are you modelling, watch parts, electronic circuit boards? I don't know about setting the visual grid, I never use grids. But anyway, using the visual grid means you are eyeballing, why would you need tenth of a millimetre accuracy if you are eyeballing? 

Look in wings prefs>constraints, you'll see various keyboard combinations to 'snap' the tool distance readout to various increments. There are also 'alternate distances' which look like they might be conversions from inches/feet etc (or they could be customised for that purpose anyway)