Fans in computers are used to suck hot air out of the case and allow proper airflow in one way and out the other. If you stick a fan in there to blow on to your components, it'll surely keep things cool, but you're also exponentially adding and trapping dust all over the place. Worse than what already happens in a normal config. I've had configs where I did just that though and is fine so long as you maintain your system. I.e.: open it up and vacuum the dust out regularly. Wouldn't use a can of air spray without a vacuum hose either though. Unless you're fine with blowing dust from one place to another, like your lungs or other parts of the room and computer. 
I'm pretty sure a small fan right above a CPU won't make any difference on case air flow. The issue I've personally seen that causes havoc with case air flow is when the case fan is sucking air in and out and another (or more) large separate case fans are doing the opposite!
Rule of thumb: make sure your fans are sucking and blowing the same direction (& no I'm not referring to rock band groupies)
