I have all my photos on a separate disk to the operating system on my PC, and copy them weekly to an external drive which I keep at my parents in laws place. I have 10,000's of photos that I would be gutted to loose, ever.
It depends on how you feel about loosing your photos. A while back I made a schoolboy error and was cleaning the back up drive completely and the "live" drive started misbehaving which meant that I could not access the photos. I felt physically sick at the prospect of loosing it all! Luckily it was just a glitch and I got it all back, but I will never risk only having one copy again!
Backups are always overrated until you actually need to use one. Disks are so cheap these days there is really no excuse for not doing it.