Right now we just render admin_level=2 to admin_level=4 boundaries (countries and states).
Other maps take a mixed approach to this. Locally, all the paper maps I have for navigation have city boundaries marked. Looking at webmaps, I see that these maps don't:
- Google,
- Stamen Terrain,
- Baidu, and
- HERE WeGo
and these maps do:
- Bing,
- OSM Carto,
- Mapquest,
- ESRI, and
- Michaelin