Here's a small guide about which Mac OS version a supporter should focus on:
OS 8/9:
Question of legacy support. Most mainstream software vendors stopped their support 1-3 years ago.
OS X 10.0
Nobody should use this version, because every Mac running 10.0 also is able to run 10.1. Plus, the update to 10.1 is free.
OS X 10.1
Nobody should use this version, because every Mac running 10.1 also is able to run 10.2. However, the update is not free. But 10.2 is a big step forward in stability and usuability.
OS X 10.2
Most Macs running 10.2 could upgrade to 10.3. Whenever possible, I recommend it, because 10.3 is a performance improvement. Installed on maybe 10% of all Macs in use.
OS X 10.3
Some Macs running 10.3 can't upgrade to 10.4 but that's okay because 10.3 does the job. Performance under 10.3 and 10.4 is roughly the same. Installed on maybe 30% of all Macs in use.
OS X 10.4
Installed on maybe 60% of all Macs in use and Apple's newest OS until spring 2007.