It's a myth. Not only is there no need to repair permissions prior to system updates, but there hasn't been any reason to regularly repair permissions since at least 10.3. Back in the day, there were a lot of bugs and/or poor code that required way-too-frequent permissions fixes. That was why repairing permissions was always considered a panacea. However, that was a long time ago and Mac OS has come a long way.
If you recently installed a third-party app and have some issues, sometimes repairing permissions can still help, since third-parties installers can be poorly coded to leave system files with the wrong permissions. But that's pretty rare today. Used to be a regular problem with MS Office, Adobe CS, and HP printer drivers, for example, but not really anymore.
Furthermore, usually the few permissions issues that you'll see fixed nowadays are minor and irrelevant. Just because the permissions are different from what the OS thinks they should be, doesn't inherently make them problematic.
Bottom line, IMHO, no reason to repair permissions before a system update. And no reason to do it regularly; it should be used in troubleshooting, not routine maintenance.
