Q: final cut express 4.  Should render files be saved or deleted?

Usually I'll save all files related to a project for three months after the client has accepted the final edit - too many times someone up the corporate chain wants a minor tweek (of course I charge for those).

But once I'm ready to clear some space, I delete everything except for the project file itself; render files, media, etc.  However, I do have several clients that routinely want to update update their video from year-to-year.  For those clients, I transfer all media files (video, audio, graphics, etc.) to a bare drive connected to my Mac via a drive dock.  When they're ready for their next update, I pop in the drive, transfer the files and I'm ready to edit.  I keep a separate drive for each client - the cost of the drive is figured in to my fees.

-DH

You can safely delete all of the render files, cache files & waveform files - they can always be re-created as long as you have your FCE project file and the associated source media files that you used in your project.

If you archive a project all you need to keep is your FCE project file and the associated source media files.  There is no reason to keep any render, cache or waveform files as they will only take up disk space and will no longer be usable if/when you reopen the original project.   (Because typically in archiving you are copying the files to another hard drive and doing so breaks the relationship between the FCE project file and the render files.  And although there is a 'reconnect media' function in FCE in my experience it does not work well if at all with render files.)