When the edge of the toposurface is visible in plan, I don't want to see a solid line there - it looks funny and could be confused with a contour. Setting Topography projection lines to White will also make all Subregion edges white (which I don't want). WORKAROUND: Use a solid white Filled Region or non-rectangular Crop Region.
A V/G subcategory for Topography edges ("Boundary Lines" or "Exterior Edges") MIGHT also work (since I'd never want to see them), as long as it had a separate subcategory for "Subregion Edges" that could be left on...
Also, there was a situation recently where I wanted to selectively hide subregion edges (but not all), so Linework would be a better solution than V/G categories (though both would be welcome).