yeah I wouldn't trust it while I was sleeping.....what if in some weird freak incident it got too hot caught fire and burnt your house down! Now thats an extreme scenario but still man, if you want to overclock you need to sit and watch it carefully. aaaand you'll KNOW when you get artifacts...if your screen is clear..you don't have artifacts...simple as that..if you got little spots all over the place obviously you got artifacts.....
To fully answer your question though it SHOULD stop before problems occur, thats the whole point of the tool...to find the max without burning out your card...but like I said what if it reached a top point and got stuck or something weird, I don't know I'm paranoid, but yeah I wouldn't just let it go while you were asleep.