After sending my D7000 to Nikon for repair of back-focusing it came back improved, although I still need to dial in -15 and -17 respectively for the two lenses I use on this body. See my earlier note on this.
I had a telephone discussion with one of their techs today and learned the following: 1) As long as using the autofocus fine tuning produces acceptable results, the camera is considered to be within specification. So any fine tuning setting between -20 and +20 that enables acceptable autofocus is within specification. 2) The lens-specific autofocus fine tuning OVERRIDES any general default autofocus fine tune settings that the user dials in. This agrees with my own tests that I did. There has been some speculation on this forum that autofocus fine tune values set for default and lens specific are added to each other for a total potential of -40 or +40. Not so. Nikon says the lens-specific fine tune settings take priority over the default values.
Based on my own experience I would advise the following: If you have a camera that back- or front-focuses to the point where autofocus fine-tuning cannot correct this, send the camera to Nikon for adjustment. It's such a great camera otherwise that there is just nothing else out there that I would prefer for the type of photography I do.