I feel that there is only one rule to obey: version numbers should always increase, but whether you do so with every change to a module or its parts is entirely upto you.

I tend to change the version number only when I publish a module, script, ... . As long as I'm the only one to use it, I don't care. The version number is mostly used to track down which version someone else is using. It helps in debugging and installing modules, etc. Other than impressing the PHB (Chief, we are now running version 5.7.12.c-(beta) of the widget-tracker and tomorrow we install the all new version 8.13.1-gamma-public) it serves no other purpose IMHO.