I don't mean a person's individual pokemon. I mean the pokemon along the routes.

So you could stay right outside of New Bark Town and battle five lvl5 kajillion ratata to level up, or you could go further down and battle something that's more difficult.

So as long as the Grunts are battling with their pokemon, they'll still level them up. Where they are won't affect their personal pokemon, just the ones they fight.