First of all, whenever you think about backing up, start by considering what impact losing everything on your hard drive would have on your life. If that wouldn't bother you, don't bother backing up anything.
Mark my words: There will come a day when you'll regret not backing up at least some files. Surely there's at least one file on that disk that would be missed.
Only you can decide if the source files associated with your project are important enough to back up. Imagine for a moment that your hard drive has vaporized and is gone forever.
Did your iMovies take a long time to edit and render? If so, back them up. Or, if you still have the original footage on DV tape, consider whether it would it be easier (or cheaper) to re-import and re-edit the movie or movies.
Which brings us to our first specific consideration: Do you need to back up all or some of your source material? And, if so, how often should you do it?
In addition to backing up the source files, you need to consider how much time you've invested in the iDVD project itself. If you had to, could you re-create the entire DVD from scratch? And, more importantly, would you want to?
If you have a lot of time invested and don't want to redo all that work, back up the iDVD project file as well as the source material files.