Using rare kitchen is like a preference. I prefer to get all my Pokemon to lv. 50 before I use them, but that's just me. If you find rare kitchen is easier to level your pokes up to 30, then you should do it. However, if you believe it will be a burden on your festival coins, prevent you from learning a move (if it does that, but even if it does, you can easily get heart scales), or it'd just be easier to train, then don't use it.