No 2 cpu's run at the same voltage. At the factory they are tested and a stock voltage is burnt into the CPU, at least with AMD CPU's it was, so there is no magically voltage as everyone's will vary slightly.
Less voltage the better.
Example I've overclocked my 2600k i7 to 4.5ghz, using -.015 offset in bios. IE I am using less than Stock volts to get this overclock.
Its been rock solid stability wise.