If you are running 10.3 or later, the file system is journaled, by default, making the manual use of 'fsck' virtually obsolete. If your filesystem does become corrupt (kernel panic, power failure, etc.), the filesystem journal is used to reconstruct it. If it can't, you probably have a bad disk or a serious low-level bug, neither of which are likely to be repairable by 'fsck'. It certainly doesn't hurt to try running 'fsck' in these circumstance, it's just that you may well be SOL, at that point.