People ALWAYS use the 3-2-1 rule for ANYTHING important. I really mean important because, as frustrating as it might be, I can re-download a lot of stuff like games and iTunes content even though I have them on my NAS.
3-2-1 rule:
3 copies
2 different media
1 offsite.
3-2-1 rule:
3 copies
2 different media
1 offsite.