Take religious (bonus is culture conversion cost) and influence ideas (there is a religious + influence policy) optimally be a country with culture conversion NI.
Russia can get 75% reduced culture conversion cost that way. For them it is very much worth it to convert culture.
Also the extra manpower and tax isn't the reason why you culture convert. It's a way to get rid of annoying foreign cores (Perm as Russia for example), dangerous rebels (no more seperatists who might break away territory, only peasants.) and making sure your cores stay longer (and you can't be forced to revoke them) in the worst case.