When you need pixel precision, you generally would zoom beyond 100% on non-retina displays. The only thing that happens with retina photoshop is that at you need to zoom in farther to get the same detail. There is a potential problem for people designing for non-retina screens, as it would appear lower quality on the actual device; But this could be worked around with existing pixilation tools, or a more general option of rendering the image at a given pixel density relative to the display.