There is nothing spiritual about ass kissing. Truth and honesty are what is required. Whenever I see someone in a spiritual context who behaves much differently that they do when I see them on the street, I know they have not opened themselves up to true transformation, but rather are using spirituality to defend their ego constructs. That may be the greatest misuse of spirituality that exists.
Spiritual growth calls for truth and honesty. Of course, there are ways to be unkind while speaking the truth, and that is never necessary. Being less than honest is worse, because it deceives the other person and therefore never bears the truth.