I would never upgrade an OS because it involves too many changes to an existing configuration that can potentially cause problems, whereas patches are designed to be deployed on an existing OS with minimal impact.
If I do need to upgrade an OS, I usually choose to install a fresh copy and migrate the customisations and configuration from the old copy rather than attempting to dump an upgrade over the top of the existing OS. This also means I have an easy backout option if things don't work out.
It's personal preference really, it's possible that OS upgrades work very smoothly these days, but I still don't trust them.
It also depends on how much you have customised the configuration of the OS, if it's pretty much out-of-the-box then the upgrade is more likely to go smoothly I guess.
Annihilannic.