That’s a hard question to answer in a blanket fashion. It may well be that Sloan’s player is within a group and story that it is far better to have Sloan overcome her secretive nature and perhaps find a more humble Belief. In contrassssst, Sloan’s player may be in a group and story that it would be far more enjoyable to entrench secretive habits and truly revel in the truth of her Belief–perhaps even the Guard needs Sloan around to save them from themselves.
In my first response to the thread, it maybe seemed that I suggest a mean PC must change to a nice PC, a lazy PC must change to a dutiful PC, a cowardly PC must change to a heroic PC, etc. However, I think that misses the point of a character-driven game. I think it is simply most important for the protagonist to change–even if they change not themselves, but the world around themselves (hello, Cpt America). That might be an entirely other thread topic.
While I always reveal the Oath and Duties of the Mouse Guard to players with the declaration, “Your Guard mouse knows this oath and has spoken it aloud at least once in life; these duties were understood even if not well understood at the time of volunteering for the Guard.”
I feel it helps to provide a good mindset for the campaign ahead, but I can imagine that it isn’t the only way to present MG.