Negative or positive airflow is dependent of your case.
For my case, positive airflow cools better. But I have really low ambient room temp as well.
/e On most cases, negative airflow will starve your CPU and GPU for air.
If your case has perforated areas like this one...then it really doesn't matter whether you have positive or negative airflow.

However, perforated areas on the back of the case (red dots) are not the best place for them, as the back of the case is where the hot exhaust from the vid card and PSU are.
If you have an un-used perforated area where the (yellow dot) is, then you can use negative airflow, w/o starving the vid card or CPU for air.
You need to think about your case in particular when designing the airflow.
This post was edited by Ghot on Jul 25 2016 07:15am