i'm in your conscience
Yeah, but I mean, if you've already accumulated all the EVs you can with that Pokemon, then a Rare Candy won't hurt. So for example, if you caught your Pokemon at level 5, I'm sure that around level 55 it's gotten all the EVs it can have and using a Rare Candy won't make a difference. I tend to just wait to use all my Rare Candies before the E4 so that I won't miss stat gains, but I mean, one harmless Rare Candy isn't gonna hurt you really. Unless you're EVing the Pokemon, I wouldn't worry too much about using a Rare Candy.
tl;dr Pokemon get more stat gains by battles, but a Rare Candy is still a nice treat and I recommend saving it till the end of the game for any level boost you might need. :)