According to the information below, I am led to believe that it is not safe. However the lens manual trumps anything, so if the manual says it's ok, I'd have to assume it's ok.
Nikon terminology is confusing. There are two similar sounding modes:
- A/M, which is autofocus with manual override (the one you want), and
- M/A, which apparently is the ability to switch between A and M with little or no lag
The 18-105mm does not have A/M mode, which is the one which allows for manual override. It may have M/A mode, as the specification seems to indicate. This is not the same thing as the A/M switch you see on other lenses.
According to this thread, Nikon support have indicated that while you can manually override focus when in A mode on the 18-105mm, you should not do this regularly.
...there is an amount of movement on the focus ring to allow you to
also focus, however this should not be done on a regular basis