If its not stable on 8hr prime and 20 runs IBT, and passes memtest, it doesn't leave my bench. I sell 2 or 3 overclocked systems a year, those leave with 12hr prime and 50 runs IBT and much more testing, but I sell many many more stock systems. I have caught issues countless times doing this on the stock setups mainly. Small things like crappy ram or a bad PSU. Even a couple cpu's from AMD and Intel doing weird things and they ended up getting rma'ed. If a overclocked system can't pass 12hr prime, it's junk overclock in my book. Flame suit on!
All you need is some mom backing up her pictures on her fancy new machine her husband said purchasing from me for a small premium was waaay better than Dell. Then while making zip files of stuff to send to relatives, it crashes and causes her to loose data. That wouldn't be a pleasant phone call for me.. Everything should be tested!