When to use all upper case?

From my understanding:

  1. It had been thought all uppercase (e.g. THIS IS SCREAMING) was hard to read because of the way the eye works.
  2. It is now understood that all uppercase is hard to read just because people aren't used to reading it.

Though this is all academic. When should upper case be used? At the highest level heading, in all levels of headings or what? What about using all upper case in bodies of texts, for example an important sentence?

I thought the benefits were obvious but no ones mentioned them. They are to grab peoples attention and add another tool to achieve levels of visual hierarchy.