The general maxim is that the lowest shutter speed you should use is the reciprocal of the focal length. For an 85mm lens this would be 1/85 second. To test this I prefocused a D700 with a 85mm f/1.4 lens on a fence gate about ten meters away using live view. I then quickly made ten exposures at 1/80s, another ten at 1/160s and a final ten at 1/320s. The apertures were all between f/4 and f/8.
The test was repeated with a D7000 DX frame body. Since the D7000 has a significantly denser pixel pitch it is expected that higher shutter speeds may be required.
It would appear that with a 12MP full frame camera a safer shutter speed is the reciprocal of double the focal length, at least for 85mm. As expected, the D7000 was more demanding, and even the reciprocal of double the focal length did not guarantee 100% sharp results.