After reading Getting the Most Out of PNG on Coding Horror... I thought I'd give it a go. That is, try out the PNGOUT tool that will re-compress PNG images files; but not for the images on the website, on the PNGs in our help file.
These are the results of running all of our 882 PNG Images in the help file through PNGOUT:
Before PNGOUT 13.2 MB
After PNGOUT 12.3 MB
(Unlike Jeff, I used a short project in Automise instead of a batch file...made it easy to pause the compression when I needed extra cpu cycles for other things :)
So that's a saving of almost 1MB. Not too shabby if you consider that this would directly translate to about 1MB shaved off the installer!