From what I understand, in the 5th gen, you don't have to wait until your pokemon levels up to see the effects of your training. You can see it after each battle.
Just try it out! Make note of your Pokemon's attack stat, then go knock out a few Patrats, maybe 5 or 10 or so. Check your Pokemon's attack stat again, and it will have risen.
Makes it easier to EV train lv100 pokemon.