Advantage?
None, doesn't matter as far as I have ever known.

I would guess if one only requires the indent bump you could start with white and work down through the grey shades as the bump depth would be based on the relative shades

You could, though technically the white would be trying to affect the look of the polys, it might not matter at all.

I am used to starting with a perfect middle gray as I also work create Terrain and Displacement maps and starting with white would not work. Keeping the same rules for all the maps is just easier for me, mentally, lol.

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