There is no "safe" voltage other than the stock voltage. Anything above this and you potentially run the risk of damage, although as Kelainefes says, this risk is minimal for the short term.
As well as being careful with the
PLL voltage, also be sparing with the
VTT. Both of these, if used generously, will degrade the CPU over time.
So, when you overvolt, there is always a risk, however small.