12.5v is fine, but as you get close to 13v i'd start to get worried, as not all components can tollerate that kind of voltage increase and it may reduce the life of others.
I guess it's judgement day then I used to run my system at about 12.6v on the 12v rail as I noticed a considerable cooling increase with that extra few RPM on both of my water pumps.
Thanks for the replies guys... It seems that I will be ok... I have just heard some rumors around about Ultra PSU's doin similar things. And I am referring to the sensor not a dmm