I backup the IMPORTANT things to DVD that I cannot replace.  That's about 7 DVDs, however photos are the ones I backup 3 to 4 times a year, but I have all the photos on two computers as well.  Paid software gets a single backup and iTunes gets a backup a few times a year.  If it's on the iPod or my daughters PC, I'm doing good, plus I backup her iPod music as well.
Of the data that I can afford to lose:  The rest of my NAS goes to full backups of the house PCs (Full weekly and Full Monthly backups of 4 PCs) and I have some ripped movies that I can create again if needed.  So long as the PCs are working fine, losing the backup is no big deal.  If I lose both, then I need to build from scratch which does have it's benefits although I don't care to spend that much time rebulding a machine.  Since I got my SSD, everything is fast.
As for how my NAS drives are used, I place backups on both drives (every other month is on a single drive, every other week is on a single drive) so a single drive failure causes no major data loss.  One drive is used for movie files and the other drive is used for any other software I like to keep handy.  This keeps the original software off my SSD.
Anyway, that's how I use my NAS and backup my data.