Auto often works, the problem is that it usually raises the voltage much higher than is necessary for a given overclock, increasing heat and reducing processor life. For example, my i7 at 4GHz needs 1.35V to be stable, while on auto, the voltage jumps all the way up to 1.47. If you manually tweak the voltage, you can get a higher stable overclock and run cooler at the same time.
That having been said, as long as the voltage doesn't exceed 1.3625V, and the temps are fine, the auto voltage won't damage your CPU or cause problems. It just isn't ideal.