When should I run RPGFIX.EXE?

From OHRRPGCE-Wiki

The only time you need to use RPGFIX on an RPG file is when you get the "Obsolete RPG file" warning when trying to load and play someone else's game. You never need to run it on your own game, since opening and RPG file in the latest version of CUSTOM.EXE and re-saving it will also update it to the latest version.