In the last seven months of watching rounds be rechambered and extracted daily I have found the following:
Of 364 rounds that were chambered and extracted daily for over a year (only about 20 of which were actually rotating through the chamber), I personally removed from service about 5 slightly setback rounds (noticeable to me, but minor) and about 5 catastrophically setback rounds (would have definitely destroyed the weapon if fired). These were likely due to individuals repeatedly rechambering already setback rounds.
Since the last ammo replacement, I have observed the same circumstances with rounds in service for about four months, and I have seen no setback of rounds, yet. (Note: old rounds were 9mm +P+ Hi-shocks, new ones are 9mm +P+ Hydra-shocks; IMHO I have reasons for disliking both, EDIT: Note that the weapons in question are a mixture of Gen 2 and Gen 3 Glock 17s)
Essentially, old rounds were chambered and extracted about 200 times each, without any inspection before I got to them, and they did eventually setback, some worse than others. All had substantial case wear (scratches and gouges), but extracted just fine. New rounds have been chambered and extracted about 40 times each with nothing but light case wear.
On a personal weapon, I had Gold Dot 9mm +P 124grs that were chambered and extracted repeatedly over about 2 years (I was young, stupid, and poor), of about 60 total and 10 cycling rounds, I saw one VERY minor setback. This didn't occur until after I took the weapon out of EDC.
In my experience, rounds can be recycled through the chamber repeatedly and still function safely. However, even the best will eventually setback, and you MUST inspect the rounds regularly. Ideally, all rounds available should be rotated through the chamber to minimize wear and the possibility of problems, and again REGULARLY INSPECT them.
Sorry for the long post, hopefully this is useful!