If no stat is above 13 OR if the average of all six stats is less than 10.0 you can chuck it and reroll. With our system this is rare, but it happens.
Oh, and rolls are always done in the presence of others.
When I started playing 3e, the first character I rolled up had amazing stats - the lowest was 12, the highest 18, and the average was a bit under 15; his net bonus total was, I think, +11 or +12. The second character I rolled up barely got over the you-may-reroll limit - low of 7, high of 15, average about 11, and her net bonus total was +1 or maybe +2.
Guess which one lasted longer, and it wasn't even close?
Hint: if you guessed the first one, try again. That second one lasted seven real-world years and might well be the most fun-to-play character I've ever had.

The first one hung on for maybe two years, three at most (we were each running two at once, as it was a pretty lethal campaign).