Well,
It depends on what one considers to be over-/understocked. I usually go with the safety recommendations of my Fishsheets (see link below) and accept that as the baseline for what a proper stocking level is. Also, I have never seen any species that "needed" (if you will allow the paraphrase) to be crowded. Any species that usually seems to need crowding usually just requires a much larger tank--which most people cannot afford and thus compromise the fish's health and go for something smaller. Then again, I have regularly been called an idealist when it comes to tank size suggestions (for example, I do not believe that an Oscar should be placed in a tank less than 84 inches long).
IMO,
MOA