Video Editors! What do you do with your old project folders?

When I'm finished with video projects, I tend to hold onto the project folders, even after I export. While this allows me to make small changes to my video later on, my hard drive is being crushed as a result. What I want to know is... what do you do with your old project folders? Do you delete them after exporting the movie as, possibly, uncompressed video? Or do you hold onto them like I do?

Just curious...