If I remeber correctly.........
Your 12v line is not used to power critical components on the MB.
That's reserved for your 3.3v and 5v lines.
I just checked all 5 of my systems and the all my 12 and -12v are running around 12.5v check my sig, you'll see that all (except the laptop) are OCed.
If you ever open up a P.S and really trace it, you'll notice that the 12v line is also the least regulated.
Something another poster mentioned, and is quite important. Get yourself a volt meter of some kind (something decent) and learn to use it.
you can check 5v and 12v lines from any HDD power molex(plug)
Yellow and black (adjacent) 12v, yellow being positive
red and black (adjacent) 5v, red being positive
If you're reading voltages from MBM4(5), bios, ect, you will not get a true reading as all are off by quite a bit.
The voltages that will affect your overclock are your 5v and 3.3v, watch them carefully. if they drop too much you will loose stability.
Sorry for the long post, I get long winded at times.
hope this helps