Fine-tuning AF with the D600

Hi Everyone,

Over the weekend, I fine-tuned all of my lenses for my D600.  On a side note, if you're interested in doing this automagically, take a look at Reikan FoCal (http://www.reikan.co.uk/focalweb/).  It's incredibly easy to use.  Even though the newer Nikons have been gimped so that the software can't do everything by itself, I was able to calibrate all 4 of my lenses in under an hour.

Lenses:

- Nikkor 35mm f/1.8G

- Nikkor 50mm f/1.4G

- Nikkor 28-200 f/3.5-5.6G (done at both 28 and 200mm)

- Tamron 24-70 f/2.8 VC (done at both 24 and 70mm)

All of my lenses have been optimized on the negative side of the scale, varying from -5 to -17 if memory serves.  So it appears to me that my camera is natively front-focusing.  Although this doesn't seem to be presenting an issue for me ​right now​, this is my first body with AF fine-tuning (I'm coming from a D90), so I don't know what this would mean for the future.  Does the fine-tuning of a camera change over time, or is it static?  I assume that since this is a mechanical setting, it should stay fairly constant?

Is it worth sending my body back for calibration, or is this something I really shouldn't need to worry about?