Some People recommend indeed to use the Coupons for Magatamas, that's something you have to know for yourself tho. If you want to compete with others then Magatamas are a must have, if not then you can just collect them through Ninja exam and other stuff.
Some People recommend to save your Coupons and Seal Scrolls till lv50, at lv50 you unlock the Tendo Seal Treasure that can give you all the Pains "easier" also it has some other Ninja's in it that the old Seal Treasures don't have. If you want to gamble and are okay with staying with the starter Ninja's or the ones you get for free from the Game, go for it~