If you have characters marked as anything but public, add them to the world at your own risk. Uploading them to the world may make them visible to world members, and possibly to users who aren't even members of the world.
The folders are currently set to public, since anything else remains hidden to all users, unless you hit the "view all" button, which will show even characters in hidden folders to non-world members.
To my understanding, linking to characters in your threads and posts should keep the privacy settings you have them set to. Unauthorized users who click the link should not be able to actually view the character.