They are called loops for a reason, it has to go against gravity one way or another.
There is little difference in terms of where it goes, the typically most used locations are rear 120mm, top 120mm. Then front 120mm and bottom 120mm.
It simply works better in terms of airflow at the back and top. The bottom either sucks air in from the desk (or floor), or blows hot air all over it. The front fills the case with heat. Radiators work best in areas where they become exhaust.