I have an SSD, but my mobo doesn't have TRIM support, so I mix things up a bit:
1. My root filesystem is on the SSD.
2. /var is a partition on my HDD, to minimize SSD writes.
3. A lot of other things that either get frequent writes or have large amounts of data are either soft links to a HDD partition or bind mounts
4. Oh, and /tmp is a ramdisk. (Programs which need lots of temp space should be using /var/tmp.)