You can use seperate partitions for /var /usr /home /tmp
But this is a matter of personal preference.
This is how i do it:
I run Slackware 10 together with Debian Sarge, Suse 9.1 and WindowsXP ( yes, i really do ) on one big HD on my main workstation.
So I only use one Partition for each Linux System, together with one shared Swap-Partition and several shared Partitions for simple data-storage ( backups, mp3, movies.... )
On my server, I have /var /usr /tmp /home each on itīs own partition, because of security reasons and ( mainly ) because I have never done this before and just wanted to give it a try.
I run Slackware 9.1 on this machine with no problems so far, so I keep it up like it is.
On my small Laptop ( old Thinkpad ), I have three partitions.
One Swap, one root and one for backups.
You see, it can depend on what you are doing with your machine and what your personal preferences are......
In my mind, there is no "the one right way" to do things