if dust is allowed to build up, it will <i>eventually</i> start to give cooling problems when it impacts the airflow over the fins of the various heatsinks. It is advisable to keep your system clean, but how often you do is up to you. once every 3 months or so will probably be fine, unless you're in a particularly dusty environment.
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<font color=red>The preceding text is assembled from information stored in an unreliable organic storage medium. As such it may be innacurate, incomplete, or completely wrong</font color=red>