I don't back up any of my itunes, cause well I have two working copies one on the G5 and the other on the MBP. I only have one copy of my movies, cause well they eat up space and I have dvd's somewhere.
My documents folder I have a weekly backup of which writes over and I mesh together every 4 weeks. And When I have very important stuff I keep a copy on both internal drives and on the MBP. My documents folder contains every thing I've worked on that has any importance for the past 4 years, wether its written work, photos, drawings, music, or code.
And I have install disks for all my apps. So I'm good.