Sitemaps & SEO Plugin questions

Regarding the SEO section at the bottom of a post where it says "Title, Descriotion, Keywords".

I was told by a friend that even if we leave this blank, the SEO plugin would still optimize the post by using the categories, etc.
(1) Is that true? (Just double checking)
(2) If it is true, is it best to allow it to do it by itself? Im asking because how would it determine the description. I can see it determining title and categories.

Im just trying to make sure Im doing things correctly. :slight_smile: