It will depend on a case by case (no pun intended) basis. It will hinge on many factors such as air flow direction case orientation and if there is enough room under a given case to adequately pull air in from the bottom. Usually adding fans blowing directly at the GPU will help feed it with cooler air from outside the case and reduce the likelihood of the fans recirculating hot air back into the cooler. This is assuming you have good airflow in the case to begin with. This includes exhaust as well, as you will need to evacuate the hot air form the case too. Also if you have your PC in the corner of a room or wedged under a desk no amount of fans will help beyond a certain point as the case itself is either recirculating it's own hot exhaust or it's just not getting enough fresh air to begin with.