I've noticed that that doesn't always work. User profile pictures can be NSFW and still be shown. And the 'random' posts box will show NSFW images sometimes as well.
It's something to be looked at to make sure all the edge cases are covered, so that the "show NSFW" check box being unchecked actually does prevent all NSFW stuff from being shown.
I'm not worried about it too much. It's just growing pains as the #threadiverse gets on its feet. The devs will fix it in the code, and until then the user can block stuff manually, which is what I have been doing.