I have a similar set up. Between music, movies and photos I'm close to the 15TB range. I'm selective as to what I back up however.
I don't back up commercial movies or music. I have the CDs/DVDs/Blurays that I ripped. If something were to happen to the NASes that's holding that media I can always re-rip. For movies/tv shows, I find myself only watching them once or twice, so if something were to happen I probably wouldn't be re-ripping most of my collection. What would probably need to be re-ripped right away would be the Barney/Dora/Thomas DVDs for the kids. For music it's fairly quick to rip (and even faster to download
The only things I back up are home movies and photos. For home movies I backup the uncompressed files, but for photos I don't back up my RAW files, only the jpegs. Those are backed up to external hard drives that I keep either at my desk at work or at my parents' place. If by some weird coincidence I would lose those as well, a great deal of my home movies were uploaded to Youtube (private) and selected important pictures to Flickr.
With that much data, what it comes down to for me is what I absolutely do not want to lose or can't afford to lose.