you should keep all of / in one big pot (again, excepting /boot and swap), because you never really know in advance which of /usr, /usr/share, /var, /tmp or whatever is going to need more space as you use the system over time.
Unless you run a POP3 server with quotas. Then you should put /tmp and /var on separate partitions, so that the temporary backup of the mailbox will be on a separate partition. Quotas are managed per-partition. When a user "pops" his mail, the mailbox is copied to a temporary backup, so that messages can be selectively deleted. If your quota for /var (containing your mailbox in /var/spool/mail/username) is close to full and you don't have another partition to hold the copy, you'll go over quota just trying to fetch your mail, with no ability to release the excess storage.
