Personally would save the coupons and stock up on power related items (charms, refine runes, cave keys etc.) No point having ninja if you dont have the power to back them up.

If you want to buy ninja I would recommend buying something that has more impact in the overall game (Shark Kisame, either Minato, Masked Man etc.). WB Asuma is rather situational and is not really useful outside of a 9T team