You only raise the voltage if it's unstable. If you reached the target frequency, you want to lower all voltages to stable minimums.
You never know if it's cause by a particular voltage. You can only suspect and that's kind of a feeling you get once you're more experienced in oc'ing.
Try to increase different voltages by small increments and seem how it improves stability. If there's a large improvement, you've probably found the voltage responsible for instability.
However, never increase multiple voltages a once, as you won't know then which one was too low.