Well, the better way would be search forums for discussions on your motherboard and CPU about that.
Sometimes AUTO may actually overvolt, increasing temperature. No harm in that, but you may find that attractive opportunity.
I leave everything on AUTO simply because it works well and no hassle, one push of a button for a small OC in my Gigabyte application. And I don't care if it's 65 or 70 degrees under load.
Either option will do no harm, as long as you know which voltage should be set in manual mode.