I just realized my present workflow was causing FCPX to eat huge amounts of disk storage for fairly small projects. Normally, I create a first version (much too long) then I duplicate it (with copy render files turned on)-cut it down, then duplicate it again and cut it down. I do some slow motion, some graphics, and eventually color correction. I found that the 5th version of a 5 minute video had 12 GB of render files, just in one version, almost all of it in a folder called "high quality video." So this is not a good strategy.
I could trash the render files of the early version (they can be regenerated if needed) or turn off copy render files when making an new duplicate project to edit down.
The best way for saving drive space is of course to not include render files on duplicates, and even delete any render files as soon as a Project has been exported. As you say: you can always re-render whenever you need to rework a Project again.
If you wish to preserve your render files for whatever reason you can slim down the size of your Render folder by just deleting the unused render files. In many cases this makes a real difference, especially if you have been rendering the same effects over and over again.
I think you are right. By copying the render files over several projects, they seem to grow geometrically. I duplicated a project without copying the render files and they dropped from 14 GB to 4 GB when I rerendered in the new version. The render time was reasonable at well. So I will only copy the render files when duplicating it there is a huge time pressure.
Or not worry about it knowing it will all be taken care of when archive the final. That's what I do. When it's done I duplicate the final version WITHOUT renders and create a final event with just the used files. If appropriate I keep the original camera card copies (if we generated the project) but usually the originals are kept by the producer/ company if we are acting as a post facility.