Agreed, your nitrates would have to get pretty high (we're talking 100+) before they might have an adverse effect on the fish. A study was done awhile back (though it wasn't on reef fish), and the nitrate levels that actually caused mortalities were ridiculously high (in the hundreds). Except as a matter of curiosity, I don't even test for nitrates in a QT.
As for keeping the QT running, I don't, but that's simply because it's not convenient. What I do instead is put the biomedia from my filter in my main tank's sump so it keeps a good population of bacteria. When I need my QT again, it can be set up very quickly. Now, if you know you'll be getting another fish in the very near future, that's another matter.
As a side note, I don't see why anyone would assume that there would have to be detectable ammonia in a QT. I've run many QTs without ever having detectable ammonia. I use a HOB power filter and do frequent water changes.