Many schools of thought on this. Do you want to delete any media NOT associated with your final project? FCPX let's you do that. Or, alternately, you can archive ALL of the media.

Then... You would want to figure out WHERE you wish to archive the projects. Many of us have extra HD's that we use JUST for archiving. (If the client comes back to you in a few months and wants to pay you for alterations.) These HD's do NOT have to have a fast connection (Thunderbolt) since this is for archiving, NOT editing. Thus, it's relatively cheap to buy archival storage.
It's very easy to move projects to another HD. You simply mount the disk, and drag the projects / events over.
If / when you wish to re-edit your projects, drag them from the slower archive drive to a faster drive.
I'm sure other folks will chime in with advice. There are lots of ways to manage media. And there are third-party apps to help you do this.