I use the 18-70mm in its midrange a lot from 27-40mm for full length and group portraits in the studio from a distance of about 3m, with flash and stopped down around f8. I have noticed that the eyes, which tend to be near the edge of the frame lack critical sharpness. I verified this with another 18-70 as well. These are 100% size crops straight from the camera. The left and right hand crops were with low in-camera sharpening and the centre was with no in-camera sharpening. All were shot as fine jpg at ISO 200. The left hand crop is the kit lens at 31mm. The middle crop is the same lens at 35mm and is quite soft. It was noticeably soft in an 8x12 print. The right hand crop is not the kit lens. It's shot with the 50mm f1.8 Nikkor lens, stopped down to f8 for comparison. It is a very sharp lens.
I don't recommend the 18-70mm lens in its midrange for wide angle portraiture with large prints in mind. The 30-40mm range seems to be the worst. Try to use about 28mm if possible as a compromise between sharpness and wide angle distortion and have lots of ambient light to aid the autofocus. The lens is fine for head and shoulders shot above 60mm.