It seems to be a common misconception that a USE flag can have a value in pkg_postrm different from its value during installation. See, for example, bug #181718 comment #3, bug #182633 comment #7, and bug #182639 comment #3. Therefore I think that some note about this should be added to the Devmanual.
Created attachment 122972 [details, diff] Proposed diff for general-concepts/use-flags/text.xml