On the B&W, I have one partition holding 8.6 and 10.0.3; one holding 9.2.2 and 10.1.5, one holding 10.2.8, one holding 10.3.9, one holding 10.4.11.
The beige G3 next to it has 8.1 and OS X Public Beta on one partition, and 9.2.2 and 10.2.8 on a second. (I didn't have room on the B&W's main drive to make a Public Beta partition, and wanted to save the other drive for data only. Not to mention the fact that I'd have to remember to turn the clock back before booting into Public Beta. On the beige, I just keep the clock set to Y2K. Which reminds me, does anyone know of a hack that will let Public Beta work past its expiration date?)
I've been considering putting 9.0 on with 10.2, and 9.1 on with 10.3 on the B&W. I'd leave 10.4 all by itself, though.
My general idea is that I like to have the first and last OSes a machine can run on it; and I like to have the slowest machine, the fastest machine, and the 'fastest at time of release' machine that can run each given OS. I classify 7.0 and 7.1 together as "System 7", so I'd want a Plus and a Power Mac 8100/80 as the slowest and fastest machines that can run it; and a IIfx as the fastest machine the day System 7 was released. (Luckily, I already own all three.) On the Plus, I keep a floppy of "System Software 1.0", since it can run it, and have 6.0.8, 7.0, and 7.5.5 on an external hard drive. On the Power Mac 8100/80, I have 7.1.2 and 9.1. On the IIfx, I have 6.0.5, 7.0, and 7.6.1. (i.e. I like having "benchmark machines" for each OS, as well as "benchmark OSes" on each machine.