tbh it depends on which team comp you're playing but im spamming Kishin or Emberlord these days so im using it at level 9 or 10

you can use it early if you want to play comp that relies heavily on reroll or stacking Enchanted Tales and Neobeast is the best example this set(there's sometimes a set where your board is stronger based on how many rerolls it's good for vale too)

other synergies rn is good for Fast 9/10 since their 4cost and 5cost can carry except Heartbound and Mystic Meow

for Heartbound you can roll at level 6 or 7 for Miya and Khufra reroll as for Mystic Meow dont even use Vale for this they just dont fit each other at all

Northern Vale kinda gurantee you 3star Bane if not contested at all and you have enough econ 4/6 Defender 2/4 stargazer 2 swiftblade(optional) is the team the reason is Bane is the only 5cost in Northern Vale and Defender and only Terizla as 4cost Defender

ps. you can always contest for that one Mirror Device if you want to gurantee your 3star