My rule is to sit as close as possible to the television, but far enough so I don't see the pixels, to have the maximum picture quality.
According to some studies, a normal 20/20 vision human can distinct something 1 arcminute apart (1/60 degree). For a 1080p 16:9 television, this equals to a distance roughly 1.6 times the diagonal length of the screen. So for a 70", this means 9.3' of distance.
Beyond that, you start loosing the full resolution advantage and closer to that, you will start seeing the pixel.