I don't think there is a set rule, just guide lines. I have tanks that are "overstocked" (only made possible by heavy planting) and understocked (what else do you put in with a GSP?).
I never let my nitrates get above 10ppm, if they are higher then it is not anywhere close to most (I said most!) fishes natural habitats. Fish are already dealing with the presence of ammonia (even though our tests don't read it, it's there in minute amounts... again, not usually found in nature) and just because a fish can live in it, doesn't mean that's what is right. These unplanted "monster" tanks get up to 40-50ppm weekly... and you house doesn't need to be cleaned every week does it? But you still do.
There is a more complex biosystem in nature than can be found in the home aquarium, that's not really an opinion; this is the reason why we need to underestimate ourselves. The best book I have found to date on this was written in 1962 (rev. 1981) by Dr. Herbert Axelrod and a few other scientists called simply "Exotic Tropical Fishes". There is a lot of outdated information and icthyology, but these are scientists that were dealing with inferior technology. If we apply today's technology to their guidelines, we can do a better job of what we got into this hobby to do (hopefully): give our pets the best life, home, and environment we can.
.02 at most
