To be honest, this is a difficult question to answer. Technically, you should never need to reinstall Mac OS X, but there's also no reason why you can't.

There are basically only two scenarios where reinstalling the operating system will likely solve a problem: Namely, if your system files have been so heavily corrupted — by a faulty hard drive or a rampaging virus, for example — that you can't boot Mac OS X at all; or if the operating system displays the death-dealing kernel dump on a regular basis. A kernel dump displays several lines of unbelievably technical UNIX terminology on your screen, usually overwriting whatever's on the monitor at the time, ending with the option to Reboot or Continue. (This is analogous to the infamous Windows Blue Screen of Death.)

If you receive kernel dumps on an ongoing basis, something is really, really wrong. Make sure that your documents are copied to a rewriteable CD or network drive and don't overwrite any existing backup that you have with a new backup because the back-up application is likely to lock up as well.