The ideal is to have the fans at the bottom pulling in the cool air that sinks to the bottom of the room, and then fans at the top of the case blowing out the heated air in the case that rises up. And then let physics do the rest. If we remember our laws of thermodynamics, we know that heat goes from hot to cold, and nature seeks an equilibrium. So as the hot air is removed, it will cause the cooler air to be pulled up to take its place, which then leaches some of the heat from the components inside the case, and is ultimately expelled from the case. And you want to have as unobstructed a path as possible for the air, so that that warm air isn't forced back down into the case. There's only so much heat the air can hold, so hot air isn't going to do you a lot of good for cooling.
Just try to mentally picture the air as like the water in a small creek or stream. If you've ever seen like a downed tree branch or some other kind of debris causing some of the water to flow back upstream or just sort of form into a little pool off to the side, this is NOT what you want. Think of cables like debris sitting in the stream, and for optimum water flow, you want it all as far to the side of the stream as possible.
Also, while the ideal is to have fans that move the same amount of air at top and bottom, if you're going to have a fan that moves faster or just pushes more air, put it at the top. Another law of thermodynamics is that as things are heated, they expand. So the cool air being pulled in is denser, or thicker, than the air being blown out at the top. So the fan(s) at the top won't be blowing quite as much air out because the molecules of air have spread out. It should happen that to keep the pressure equalized, the lower fans will only be able to bring in the amount of air that has been expelled, but as was pointed out, cases aren't sealed vacuums. So better to have a little oomph at the top, but don't go putting some ultra high volume (of air) fan on the top and then cheap low volume fan on the bottom or you'll burn out the bottom one as it's forced to speed up to try and match the high volume one.
If this is getting a bit complicated, just remember a couple of simple laws of physics. Heat goes from hot to cold, and for every cubic inch of air you pull OUT of the case, another cubic inch of air has to be brought in. So making sure you have the same amount of airflow at the top and bottom is key. If the top fan can move 30 cubic feet of air per minute for example, make sure any combination of fans at the bottom of the case total up to 30 cubic feet of air per minute.