Your experience may be a good deal more trouble free, but after my first attempt at running OSX final ended with my OS9.1 partition hosed, I wanted to be better prepared next time.
->4gb partition, my main OS9.1, with a vast assortment of application.
->4gb partition for OSX
->10gb Documents partition, featuring a read-only/compressed disk image of my main OS9.1 partition, including applications.
->650mb partition, with OS9.1 and the bare minimum number of applications that I have to have available via Classic.
When I need to use OS9.1, and not Classic, I need to in a hurry. Taking a day to put Humpty Dumpty back together again is not something I've got time for. Equipped with a custom disk image and my trusty Apple Software Restore application, I can restore a clean, fully functional OS9.1, complete with applications, wipe the offending OSX partition, and be back to work in under 20 minutes. Knowing that I can do this makes the sometimes bumpy ride of OSX a good deal more enjoyable.
My disk image weighs in at about 900mb. In order to use Apple Software Restore to restore this image, rather than the image that my Mac shipped with, the disk image has to have a script run against it. The script (Scan Image for ASR) is available as part of the Disk Copy package supplied with the Installer SDK 1.2.4 (and available via FTP from Apple). Complete instructions are available at macaddict.com.
Among it's features, ASR allows you to reformat a single partition, and as such can also be used to wipe and restore something onto your OSX partition. Note that OSX will leave behind an odd assortment of files which are difficult to remove. ASR makes quick work of this, without requiring the reformatting of your entire HD.
As a final note: by the time you're faced with having to do this, you may be unable to boot off of any OS9.1 partition, and may have to resort to a CD. If you have a TiBook, and haven't heard yet, the OS9.1 that shipped with OSX will not boot your Mac. You have to use the System that shipped with it....