I thought this point had been raised long ago but I can't find a previous thread for it. It was raised again recently  by a user reporting to the helpdesk. When very zoomed-in, the scale bar steps in sensible steps to integer miles (or integer tenths of mile if very zoomed in. But as you get further out, the bar still steps just to "integer miles" values, but integers stepping in 10-mile then 100 mile steps would be neater (e.g. 0.1, 0.2, 0.3, ... 0.8, 0.9; then 1, 2, 3, ... 8, 9, 10 as now, but thereafter 10, 20, 30, ... 80, 90, 100, 200, 300, ... 800, 900 - or perhaps even 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, 500, 1000).

NB, this ONLY refers to the physical length of the ruler bar and the number written at the end of it, it does not relate at all to how the zooming scales the map.