What's included in this update:

Upgrades Safari to version 4.0.2. 
Improves the accuracy of full history search in Safari 4. 
Resolves an issue in which certain resolutions might not appear in the Display pane in System Preferences. 
Dragging an Aperture image into Automator now invokes an Aperture action instead of incorrectly invoking an iPhoto action. 
Resolves an issue that could prevent importing of large photo and movie files from digital cameras. 
Improves overall Bluetooth reliability with external devices, USB webcams and printers. 
Addresses an issue that could cause extended startup times. 
Improves iCal reliability with MobileMe Sync and CalDav. 
Addresses data reliability issues with iDisk and MobileMe. 
Improves overall reliability with AFP. 
Improves overall reliability with Managed Client. 
Improves compatibility and reliability for joining AirPort networks. 
Improves Sync Service reliability. 
Includes additional RAW image support for several third-party cameras. 
Improves compatibility with some external USB hard drives. 
Includes latest security fixes.

What is the rule of thumb for installing systems updates like this one?  

Thanks!

Not just a rule of thumb, but pretty much a posted guideline.  You should wait to update until you see publsihed support either here in the forum, or on other areas of the Avid Support pages.  Meanwhile, watch these forums for reports.  If enough users have a trouble-free update experience, go for it.

Thanks Randall for the quick response it is appreciated.  I will sit tight.