I've done it on my G5 & many G4s, no problems.
"Try Disk Utility
1. Insert the Mac OS X 10.5.6 Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
One more secret...
1058 Combo Update, 10.5.8...
Repair Permissions afterwords, reboot.
In fact you have to Repair Permissions twice in a row after applying that combo update.