It tends to be related to the process. Intel's 65nm process is good for around 1.6V or so, Intel's 45nm is around 1.4V max, AMD's 45nm is around 1.45V-1.5V max IIRC, and I have no idea what the max for AMD 65nm is.
For some reason it's hard to find that info on 65nm AMDs. Or at least the Phenom II 940. Recently talked to someone here, info might be useful. Guy was OCing a Phenom II 940:
Well 1.325 is the area where AOD starts to turn red. As far as experimentation. I have change bios settings honestly about 115 times. lol. Its still running good. I was just looking for safe 24/7 final voltages.