This is another way to look at itactually, its better to trainer a Pokemon from the moment thier hatched, there you can EVs Train. when you reached the maximum amount of EVs you have earn which is 510. you wont be able to recieve any more. from then on after when training is complete, its a hassle to Lvl them up afterwards in some occasion, so Rare Candies are in a welcome comity when we want to. basically, its not good to use Rare Candy on a Early State because the Evolution from its previous state could increase and multply when the Pokemon Evolve, which of course does not effect pokemon that does not have a Prevolution or a Evolution.
Ev training can be done at any time is the point whether or not you have used rare candies