For what it's worth I've installed QT 7.3.1 and so far, no problems.
However, I never ever let Software Update install anything on my computers.
I always select "download only" from the UPGRADE menu and download and install it myself.
Whenever there is a system upgrade I always go to the Apple website and download the Combo updater and install it myself.
Also, I repair permissions before AND after each installation.
Sounds like a lot of time consuming work but I can't quibble with the overall results.
I've been doing it this way since Panther and have never once had any problems.
Hope this helps,
Jim