You should definitely be cleaning the dust out of your tower at least every six months. It won't significantly raise your performance, but it will lower the risk of freezing and long load times due to things running hotter than they should. It's just general good computer hygiene.

If you've never used compressed air before, it's important to make sure you NEVER turn the can upside down, or hold it sideways while blowing; only right-side up, even if that means you have to move the computer around in order to get to certain areas. Tilting the can may cause the contents to come out in liquid form rather than gas, and the liquid will freeze on contact. For the same reason, blow in short bursts, not continuously, and in sweeping motions, not directed at one spot for a long time, because otherwise you may cause freezing. be careful not to get any dust in your eyes from the blowback, either; goggles or at least a pair of glasses/sunglasses while you blow wouldn't be a terrible idea.