The only gun I've owned that I felt the need to run 500 rounds through, prior to carrying, was my Kimber Ultra Carry II. It was recommended by the manufacturer, but I've also seen the same thing from some of the other "defender" model 1911s. Mine had a manufacturer defect straight out of the box, and I never fired that many rounds through it before sending it back to have it remedied.
Since the repairs, I have fired roughly 200 rounds of Winchester white box, and a few boxes of Hydrashoks, and I feel 100% comfortable carrying it now.
For any of my other 1911s, or my Glocks, I have never felt the need to fire them that much prior to carrying. I absolutely test them all before carrying, but unless I experience any issues I don't worry.
There is always the "if" factor with any gun though. No one on this forum can say that they know, with 100% certainty, their gun will go bang every time. We can all be sure, but in a world with infinite possibilities, where even the most tested and trusted gun works reliably all the time, it is always possible for something to fail. This is getting a little too extreme for the discussion though. If you find a gun that you can trust, and have tested it, barring those million to one odds, you can safely assume that it will function properly after a few hundred rounds, IF you are familiar with firearms. For the novice, I would not suggest carrying until they have tried and tested multiple guns, with multiple types of ammunition, and had some sort of training.
Long story short, I wouldn't trust anything straight off the shelf, but I also don't feel like I need to reach a certain benchmark before making a decision on what I carry. Others may of course disagree, and I think that it is wise to do so, each of us has our own personal standards for what we trust our lives with.