IMO, it is better to maintain voltage within spec for general use and core temperatures under 70 deg C. (you are water cooled, right?) I would not go more than 10% over spec (loaded Vcore) for short bragging tests. (Too much voltage increases the chances of punch through damage.) I think it increases risk to max out voltage and temperature at the same time. However, that is just my opinion, considering how much these processor chips cost and the benefit I would actually get from running it faster... For example, I know that transcoding high definition video files averages only about 60% use of four CPUs but can run for hours. So temps will be well below max and the benefit can be a much faster processing time. Then stability comes into play because the longer it runs, the greater the risk of a crash and the video file will be ruined if the system crashes... All in all, I think it is best to back off several notches, in the interests of stability and computer longevity.
Edit: You will find many crazies with other opinions here http://www.xtremesystems.org/forums/forumdisplay.php?f=56&page=2&order=desc