Hmmm...what program are you using to get that reading? What do you have hooked up to those molex connectors! Also, what brand PS are you using?
As for "is it safe", I've had boxes skirting the 13.1V range which occasionally rebooted, but are you getting random reboots? Read it again with Mother Board Monitor (MBM, and good program you can download) and post the results. Again, if you're not rebooting, IMO you'll be ok.
<font color=purple><i>If I had a notch on my belt for every PC I'd destroyed, I'd have a really crappy looking belt</i></font color=purple>