yellow pretty much sums up the rule for backups. How much can you afford to loose?
Right now, I make sure to have everything really important on my boot partition, and keep less frequently changed or vital files (for example, old video projects or scratch-disk-stuff when I'm doing video or audio work) on another partition or drive. This less vital stuff gets backed up to a firewire drive or DVD-R whenever I do something significant--finish a project, or do enough work on one that I wouldn't want to lose it.
As for the important stuff on the boot partition (mostly writing): It gets cloned to a partition of a second internal drive at 11am every day, so I have a bootable backup with no more than 24 hours data loss in the event of a catastrophic failue. I additionally back it up to a firewire drive every month or two (or more frequently,depending on what I'm doing), which lives in a watertight bag in my firesafe in case my house burns down.
Further, the REALLY important, irreplaceable text files (writing for my website, and time-consuming creative writing) are cloned to my webhost hundreds of miles away, which also has its own versioned backup system, so even in the event my house were obliterated or everything got stolen, I'd still have the most time-consuming and irreplaceable work backed up. Thanks to Archive.org, there's even an offsite backup of the public stuff there.
Yes, I'm pretty careful. I figure if I've put literally hundreds of hours into writing the stuff, I'd better make sure it doesn't go poof.