U onli overvolt when ur ram is unstable, unstable when u tighthen the timings or at high frequency...Of coz, i would suggest u raise ur voltage around 2.8 v b4 overclock.. THis can burn-in ur ram, which able to make ur ram more overclockable, some says itz a myth...

However, any voltage above 3 v is not recommended unless ur ram stated it can run at tat voltage..Also need to note that which chipset ur ram consist, as different chipset has different potential on the overclockabilty with higher voltage