Interesting topic, and for the most part I agree with those who have said polite correction is the way to go. But, I'm not so quick to jump on the " your not qualified...therefore you get what you get" band wagon. Aside from the safety concern, and situations where it is the persons (NCOs) job to correct the officer (i.e. BOTC, Phase 2).
The question I have is, where is the line drawn, if I were to walk down the hall and had obvious deficiencies (Flag on upside down, pants unbloused, etc..) then I would like to be politely corrected. For things relatively minor, scuff on boots, wrinkle in pants... does it need to be said at that time and place... if you think the answer is yes then it can be done politely and respectfully. However, if the NCO is stopping the officer in the hall and telling him his boots are very bad and than he is bag of very bad stuff that is inappropiate, and the matter must me directed to the chain of command. Because the goal is not to make the officer feel like an idiot, but rather to correct his current deficency.