The best use of Recruitment Posters is to unlock Jackbot (either using 10 Posters if you have them, or to cover the shortfall if you don’t succeed after attempting Special Summons) because he’s the Best Farming Hero and there’s no way to unlock him for free.
If you have yet more Posters, save them for Premium heroes - but don’t use the Posters until you have enough to unlock the hero you are aiming for - better to keep your options open