How necessary it is to use "must-revalidate" in "Cache-Control: no-cache, must-revalidate"

Question

Often I use

Cache-Control: no-cache

or

Cache-Control: max-age=0

The spec says must-revalidate is for max-stale... (the server issue max-stale?)

So if for normal web servers, Apache, or Rails with Mongrels, then I think usually there is no max-stale, so must-revalidate is not needed?