There is no question that over and under clocking could cause system instability that could lead either to loose of data or system hangs, so why write about this? Because system stability, as I found hard way, is also OS specific.

Common way to decrease system power usage is undervolting components so that way, even thought they do operate within same spec, they do use less power to do so. Less power equals to less heat being generated, so this also helps to decrease component temperature a bit.