Here is a good link suggested by Phos
this advice above doesn't necessarily agree with the advice below. whatever works for you and keeps your system purring is the correct advice.
Below is suggested by Ramón G Castañeda.
1) Repairing Permissions (with Apple's Disk Utility) is recommended BEFORE and AFTER any system update as well as BEFORE and AFTER installing any software that uses an installer.
Additionally, if your machine does not run 24/7 so that it runs the daily, weekly and monthly Cron Scripts in the middle of the night as intended by Apple, run Cocktail (shareware) as well.
Repairing Permissions after the fact (i. e. not immediately before and after an install) may NOT help.
2) Cron Scripts are maintenance routines designed by Apple to run on a daily, weekly and monthly basis in the middle of the night.
If you don't run them, you WILL run into trouble, sooner rather than later.
Here's an excerpt from the Apple tech doc http://docs.info.apple.com/article.html?artnum=107388
Mac OS X performs background maintenance tasks at certain times if the computer is not in sleep mode. If your computer is shut down or in sleep at the designated times, the maintenance does not occur. In that case, you may want or need to run these manually.
Mac OS X periodically runs background tasks that, in part, remove system files that are no longer needed. This includes purging older information from log files or deleting certain temporary items. These tasks do not run if the computer is shut down or in sleep mode. If the tasks do not run, it is possible that certain log files (such as system.log) may become very large.
Also, from: http://docs.info.apple.com/article.html?artnum=106978
The disk activity generated by find is a normal part of file system maintenance, used for tasks such as removing invisible temporary files that are used by the system. It is scheduled to occur early in the morning at 03:15 everyday, 04:30 on Saturdays, and 05:30 on the first day of each month.
here is some new info on crons from the apple boards