only spend them on weeks that have Fukurokumaru Deals. And even then, spend it in an event with both a wheel/board and a redeem feature (like Lucky Board, Lucky Stars Wheel). Spend for a meta ninja and then finish off the rest of the coupons for training potions til you get the max tier of the Fukurokumaru deals (or work towards another ninja).

This way you get to maximize your coupon spending. You get 2 new high tier ninjas, and a lot of power items both from fuku deals and the event you spent in.