Well, let's see how close to correct I am:
You spent lots of time on a project and accumulate all sorts of files. You successfully make a DVD. Hoorah. Now what? There's 30 gigs of junk all over the computer in a disorganized mess. If you start moving things around or renaming things you'll never get the project file to work again, certainly not in three years time.
1. Immediately make an .avi file of the finished project (in Make Movie). You can leave the DVD menu in it, although it won't work as an interactive menu anymore.
2. Save the DVD file (Video_TS folder) and the .avi file in a folder on your external backup drive.
Throw everything else away (or return it to its former place) , including the project file.
If you are content to live forever with only the final video, there's no need to save any of the source files. The .avi is good for any further editing or reuse down the road, and with no perceptible loss of quality. And the Video_TS folder will make the DVD forever, and requires no source files whatever except those within the folder.
I've stated this with great authority, and with fingers crossed. Please correct!
Cheers
Christian