* Always back up in *some way*. While a copy of the data on the same array won't protect you against all problems, it will protect you against some.
* A *backup on the same server* is susceptable to the *same failures as the original data* set (hardware failure, natural disasters, and the like).
* A good rule of thumb is *three copies* (the RAID is only one copy for this purpose): the *original*, one *onsite copy*, and one *offsite copy*. Store the offsite copy in the cloud, or at a friend's house.
* *Make backups regularly*, at least once a week; the day you need a backup is the day you realize you hadn't run it in 6 months and what you need isn't backed up.
* *Test backups regularly*, at least once a month; *a backup is worthless if you can't restore from it*. Just because you have a backup doesn't mean you're protected; always test them.
There are dozens of backup utilities out there; I'm not going to prosthelytize for any one of them, but I personally use [BackupPC](http://backuppc.sourceforge.net/) for my server and workstation backups.
