I had done using Guru3D driver cleaner only when I had 10.6 driver and I wanted to install 10.4.
1. Uninstalled 10.6 driver
2. Reboot and into safe mode- Run Guru3D driver cleaner
3. Reboot and into normal mode and run CCleaner to remove all of ATI regs.
4. Installed 10.4 driver.
This is flawless method I ever tried.
if I want to install 10.7. I would choose to install over 10.4. But then if I don't like 10.7 driver, I would click the "Roll back driver" in the Device Manager.