All Prime95 does is put stress on the CPU. If CPUs were not built for stress, then they should'nt have been publically released. I just got done overclocking my AMD FX-6100 (stock 3.3GHz) to 4.1GHz, and I've probably accumulated about 12 hours total combined time of running Prime95. It is absolutely fine. Worst thing that could happen is the computer shuts down from overheating or instability (with overclocking, I have had about 20 shutdowns! Just reboot.) Also, Prime95 commonly detects problems that other stress testers do not detect, and I've tried a lot. Go with Prime95, and you can't go wrong!:D
Edit: Also, I recommend running Prime95 for anywhere from 1 hour minimum to 2 hours max at a time. Seems to find 99.9% of instability, and maximizes efficiency.