Hmmm here are my .02...

with negative airflow you will not have as efficient heat dissapation, like xMerCLorDx, pointed out...

If you have a too positive air flow, you will get more acumulation of foreign particles in your case: Dust... so you will need to use some type of filtration on your fan...

Sry if I didn't really answer your question, but it's one of personal choice...