If it's stable under load/testing then you're fine. You can't hurt the card by undervolting. It will just crash if there isn't enough power.
I always run GPU-Z in the background as it will give you information about everything system-related, including power stability in the "Sensors" tab. Scroll down to "PerfCap Reason". If you hover your cursor over this label, a drop-down menu explains the various codes it is showing.