Another point is that sometimes white spot may seem to disappear when it's really just dropped off and is really hatching out billions more little critters which won't be seen until they attach themselves to the fish...
If a fish in quarantine has white spot, best bet is to treat immediately, before the poor fish becomes covered - this being the point of quarantine, to discover and cure any ailments prior to risking the fish in its intended tank.
Edited as I'm evidently spelling-deprived...