Imho, you should always save resources until you can get enough to get the best out of rebates and fuku deals. Meanwhile, if you can't wait, you can't go wrong with spending 3k coupons monthly on konoha outlet's training potions.

For power, coupon-wise, prioritize flags and potions. The ninja assist feature frankly trumps all the others. The other resources can be gained passively by playing the game normally and collecting free stuff anyways. If you are ever close to achieving something you want, of course go ahead and spend for it then and there.

Just my 2.