I think as a couple one should have “share-close-to-everything” kind of relationship but still keep a few thoughts to themselves. It’s beautiful to share but not necessarily everything. Sharing your thoughts, dreams sure helps to grow you as a couple but there might be somethings which are NO NO! Sharing every tiny detail might create problems. In a relationship, both should be open but there should be certain personal space. For example one can be fine about talking about their past relationships but another may not. So, it’s basically depends on how much you have grown as a couple.
Going forward, sharing should be done but sometimes it is best to leave things unsaid. You don’t need to share everything that comes to mind. There are millions of thoughts and it’s not good to spill everything. So, one should find out what their partner is comfortable with. Does s/he want to hear everything? The answer to this question will lead a couple to draw a line to keep some secrets as secrets or to open up everything. Don’t hold back sharing everything, but do so after knowing s/he is fine with it.