I used to do several maintenance procedures on a regular basis, including rebuilding the directory (Disk Warrior), repairing permissions and running maintenance scripts. I have found that 10.6.8 is the most stable OS that I have ever used (I have no experience with 10.7 or 10.8) and, whenever I check the directory (a rare occurrence), Disk Warrior finds nothing to repair. I occasionally repair permissions (maybe every other month) just for laughs and I run the maintenance scripts if I'm bored, but otherwise I'm fairly relaxed about it.
I do, however, religiously maintain a Time Machine backup as well as a SuperDuper clone.
For another point of view, read
Do You Need a Third-Party Disk Utility.
