Google+ Location Sharing: How to share only city level locations
This is now addressed in Google+ v4.1


  4. Location sharing includes more controls
  Starting today you can share your pinpoint or city-level location with others (if you want), and you can do so on a circle-by-circle basis. For example: you can share your pinpoint location with family, and your city-level location with co-workers. You can also use circles to filter who appears on your “Locations” map.
  
  It’s rolling out to Google Play over the next few days (goo.gl/xaxMx)...