Without going into alot of detail, a voltage regulator will output a curved voltage out. With no feedback at no load you might get 12V, at 10% load it might be 11.9V, at 100% it could be 9V. Obviously this is bad as 9V won't keep your computer on. The solution is feedback to the regulator. Unfortunately it's not a cut and dry set it and forget it. They have a range they have to stay in so they have to set the feedback. You are running somewhat high, but are still within spec. It could be that your PSU is actually going out of spec at certain loads. It could also be that the 12.45v is more of an indicator of something more serious wrong with your PSU, for instance the filter circuit might not be cleaning up the AC and you have a voltage ripple that's ruining your MBs. I'd say that although 12.45v is within spec, I'd be skeptical of it. Replace it with a spare and see what happens.
Also, is your voltmeter calibrated? Does it actually give an accurate voltage for the range you are using? If you are on AUTO you should be concerned with the exact range that AUTO is choosing for you.