Originally Posted by BashfulTux
The already mentioned link to Prime95 download and run it. To answer your original question there is no harm in under volting your processor if it will run error free when using the Prime95 at that voltage.
The problem I see so often is that people think they have properly stress tested their overclocked CPU's when in fact the majority really dont know how to properly do it, a lot of people end up only stressing 2 of 4 cores, or they do not run the Specified SMALL FFT's test but instead run LARGE FFT's which is mean to stress test the ram. lol.
Use the Link that I posted for P95, it stresses all 4 cores but you must make sure to select SMALL FFT's for testing. Also let it run for 6 or more hrs error free. If you can do that you are good!