Most bios have the so called "Safe Voltages" colored coded. But there isn't really a "Safe" Voltage. All you have to do is browse this forum and you'll find numerous users with overheating issues at totally stock settings.
To me, the most important factor is Heat. Monitor your Heat and Regulate it.
Some people get into overclocking and think they need a Monster Heatsink or Water Cooling so they can shoot a bunch of volts thru something and run Blistering Fast. Unless your attempting to set a World Record or a Forum Record (in which case you don't need to read my guide) that just isn't the correct way to approach overclocking, imho.
To me, the purpose of a Monster Heatsink or an Advanced Cooling Cooling System, is to overclock with as little voltage as possible. While the results might seem to be the same, I think it's what makes my overclock better than most