Myth #1: “You should repair permissions regularly.”
OS X’s permissions determine the access each user has to each item on a hard drive. If certain files have the wrong permissions, you can experience problems. So you’ll commonly hear that you need to use Disk Utility’s Repair Disk Permissions feature on your startup drive as a routine maintenance procedure.
But contrary to popular belief, repairing permissions—a procedure which simply resets permissions to a known state—works only on a particular subset of OS X system files. It doesn’t affect user files, nor does it affect third-party files or programs. In other words, it’s unlikely that regularly repairing permissions will prevent problems.
If you ever do have a problem with system-level permissions, your Mac will likely behave oddly, and you’ll usually be able to use the Repair Disk Permissions function then to fix the problem without any data loss or long-term effects. So I recommend repairing permissions as a troubleshooting tool rather than a maintenance task.