Undervolting can not harm the processor so I'd definitely go with what gives you the better CB20 score.
The reason they say not to undervolt is at a point the processor will get into a state called 'clock compression' and it will under-perform even though the clock speed still seems fast. So only undervolt it as far as those CB20 scores hold up.
It may be AMD's own utility but it really is a bit flakey with clock speed and voltage readouts I think it's doing some kind of weighted averaging nobody's been able to figure out. I suggest using HWInfo64 but look at the (SVI2/TFN) voltage reading since it's the actual core voltage the processor reports out in telemetry.