I would say it is at it’s best before True Power of Storms comes online, and when Grave cannot safely reveal Jack in neutral. In other words, it’s at it’s best in the early-mid game, once your hand is Rock Armor worthy, and in the very late game where you’ve successfully blocked or Jockered TPoS, or you’re just looking for lethal off of throw.
If you’re at a low hand size or your hand isn’t well developed, it’s almost impossible for you to deal with Grave’s Jack except by blocking it (and thus letting him build a hand at the same rate as you), or by throwing into it (just to keep it from returning to his hand). This is the state of the game in the very early turns. Losing a King to a Jack isn’t great here, when you would rather spend a 2 throw, 6 throw, or even a big normal you don’t much care about.
If Grave has TPoS, he has to be thinking of accidentally running it into a block, and especially running it into a 5 block. For that reason it’s often tempting to try to hit confirm into it by playing normal attacks or dodging. Ordinarily Rook’s King would shine against this type of playstyle, but Grave has counter’s, which will let him treat Rook’s King like any other throw.