True, Use any possible chance to get them (Ninetails ranking rewards / Group Summon / Plunders if you can or convoys / benefits hall rewards / recruiting ninjas / Group wheel if you reached the group skills max level then use the group funds in the wheel / and many other ways)

But the most important step is to know exactly what is the most benefial F2P spending event and use your coupons there and thats of course based on your items and attributes where you need to focus.