Any advantage to not using a HD to full capacity?

It's mostly a waste of time. If there is any speed increase it is not enough to be worth the exercise. If disk speed is essential to you, then use an SSD instead.

Six of one, half a dozen of another. 😉

The more free space on the Boot drive, the better the chance of the OS finding contiguos space to write.

You're welcome... good luck! 🙂

I doubt that you would see much of a performance difference, although there are some other good reasons for using multiple partitions, for different OS versions and for simplifying backups, for example. you can have different versions of the operating system, perform updates on one, and you can still boot from the other one to down-grade back to what you had, in the event of a corruption during update or a broken feature or third-party driver incompatibility,etc. I don't know how many posts have shown up here after a major OS update from Apple asking how to downgrade back to the old version, if only they had a duplicate on a partition before they performed the update.

If you are using a large external drive for backups and using Time Machine, consider partitioning the drive to make one volume a bootable clone of the OS and use the other partition for Time Machine. Time Machine will fill up it's partition eventually, so having a dedicated partition for your bootable OS ensures that you won't run out of space for that one.

At one time, maybe for the older first generation large drives there might have been some small performance boost in seeking data from a smaller volume than from the entire drive, but I doubt that with today's drives with larger memory buffers this is still the case at least not to a real-world noticable difference, but I have not done any such benchmarks myself. Maybe you can try it out for yourself.

If performance is an issue, put some SSDs in a hardware SATA RAID-0 and some instant coffee in the microwave. You just might go back in time.

Good points Glen, thanka!🙂

Sounds like you have a good idea of how to use partitioning to make your life easier. Going overboard with partitioning can make things overly complex. When I first upgraded my G3 Desktop long ago, which I no longer have, from the stock 6GB drive to a 60GB drive, I went partition crazy, I had about 8 partitions and that particular model could boot from OS 8.1, (I skipped 8.6), 9.2, 10.1, 10.2, 10.3, and 10.4, after a while the novelty of rebooting different OS versions wore out and I eventually narrowed it down to running just OS 9.2 and 10.4. (PS - I also could boot that G3 into MkLinux on an external SCSI drive, I did not want to mix Mac and linux partitions on the same physical drive)

Sounds like you are also aware that when you boot into 10.5 you won't have Classic emulation available, so keeping the OS9 System Folder and OS 10.4 on another partition will allow you to boot into 10.4 should you need to run any older Classic software.

There may be some other devices you use, like a printer/scanner or camera for example, where you have drivers for 10.4 but not for 10.5. Although, you might be able to download an updated driver for 10.5, like from www.hp.com or www.epson.com, etc, if you were using an older printer in OS9/10.4 and find that it does not function with the 10.4 version drivers on the CD that shipped with it under 10.5