I would save them for the next lucky board. The rewards in this one isn't very good unless you spend $400 for Hidan. I am currently saving my coupons for Sailor Sakura when she decides to pop up again. Honestly though the best thing to do with coupons is to get for power, but I have a tendency to * them on ninjas. The best event to most people though is Lucky Board because of you will get the most out of your coupons, plus everytime you pass start you get a gift box that has a chance to drop Hashirama. If I was you I would save them till the event comes back around in a month, but if you can't wait that long use them on power like when/if the cave key rebate comes around, that will increase your power quite a lot. Also if you ever decide to spend a little bit of money on the game I recommend getting the monthly card pack rebate. 300 ingots get you 300 coupons plus 50 coupons a day for 30 days (1500 coupons).