a dual parity scheme may seem excessive when you've only got 6 drives in an array (33% usable space sacrificed to parity), but my formula is to weigh what it costs me in time and hassle to restore all the data from a backup in the event of an array failure (or worse, your physical DVD and Bluray media is your backup and you have to re-rip them all), versus what it costs for the extra harddisk to provide a second parity stripe. extra harddisk is generally cheaper.
some people would argue that with only 6 drives, raidz with only one parity stripe would be "enough" in many scenarios, since ZFS is doing things to protect your data against from some of the pitfalls of traditional RAID5, like checksumming writes and protecting against silent bit errors, etc. there's some truth to that.