Superleccy: OS X handles file defragmentation on the fly, yes - on top of which, *nix is designed to work with a certain level of fragmentation - if you defrag OS X it'll actually get slower.
For the OP - no, I wouldn't recommend partitioning unless you have a very specific reason to do so (such as using BootCamp). I would, however, recommend the purchase of a cheap external USB or Firewire drive for backing up purposes.