lol, I usually install the I/O shield before installing anything else on the motherboard. Not quite the correct build order, but I really hate having to strip everything back down because of a forgotten detail.
Connectors for things like USB ports and audio jacks always use chassis as ground, mostly so that ground is always electrically connected before any hot pins when users plug things into them. I've seen a lot of I/O shields which use extended metal ground flanges to physically wedge themselves into place. I've seen a few PC cases which didn't exactly conform to ATX form factors and made proper fit of the I/O shield a bit of a pain.
PSU, motherboard, and chassis should all have common electrical ground for best stability. Components mounted in anti-vibration rubber chunks don't always make electrical contact, ground loops can be problematic to troubleshoot.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]