from what i saw so far since i started playing this game ( 4 months ago ) luky board is always on 2nd week of a month not first.

now to answer the OP , i would always save all coupons and spend them on luky board with few exceptions , that beeing if in a event there is a really good discount on a good ninja and you have enough coupons to buy him outright ( like gnw tenten, sailor sakura, sage naruto, and if u have the ninjas to make a full team with it ) otherwise just go for luky board only, and always prioritize power over ninjas with the few exceptions i mentioned above.