I own CS5 Design Premium, and I also subscribe to Creative Cloud. I avoid using the CC version of InDesign, because of concern about opening new files if I stop subscribing to CC. I'm wondering if there's a way to set up my InDesign CC preferences so that when I save a file, it automatically and simultaneously saves a IDML version as well
There is not a native feature in InDesign that will automatically save an .idml file when you save your InDesign file. It might be possible with scripting thought. You can however Package your file using File > Package and enable to option to automatically generate an .idml file during the package process.
In InDesign CC 2014.1 v.10.1.0 you can package your documents with the option to add IDML to the package.
Saving and exporting to IDML is also possible, but this requires a startup script (ExtendScript-Script).
Search for "Martin Fischer", "saveVersions.jsx" and "hilfdirselbst.ch" on the web.
Martin once wrote such a script that would work for you even when on CC…
It's in German, but should work for you as well.