Of course sincerity is needed. As they say, that the best cure in relationship - is deterrence. If you want to avoid some situations like: eating all your life stuffed eggs (rice, buckwheat, etc.), to have for a present the flowers you like but not the ones you have allergy, to watch movies, after which you cannot sleep all night, then for your sake tell the truth to your partner. The only thing you must understand is the difference between sincerity and harsh truth that could hurt that person. If you speak honestly about yourself, it is understanding and acceptable. But if you want to become that sort of a woman that speaks the truth and the regrets it for the rest of her life, then you should think carefully. After all, the truth can hurt people badly. It is not recommended to tell to a very sick person that he has no chances and he will die? There are miracles in our life and sometimes incurable diseases are cured. And secondly, that person knows in what condition he is and your truth will not make him happier. I think you understand the general picture here. Take care about yourself and your life, raise self-esteem and become happy.