Desaturate just erases the color data from the pixels(Period.) You end up with as fine a quality image as you started with. The adobe manual says the first thing you should do is Image>Adjust>Levels. When you open this up you'll notice a curvy black area. If you set the top sliders to the beginning and the end of the darkened area, you'll have a finer quality image(Provided it's not there already.) If you want you can open the Image>Adjust>Variations dialog. Then you're going to want to apply the Unsharp Mask. Another way to achieve a nice grayscale is with actions. There ae actions out there for light greyscale-heavy grayscale. Some of which use the Duotone, Tritone, etc. methods.