there was a thread not to long ago that answered this question. from what i can remember and one of the reasons i keep mine at the bottom is so that i do not have to unplug my heater when i do my daily water changes. other people do it so that if they do forget to unplug their heater during a WC the first thing that will hit the water level is the top which can be out of water. also, with discus who are spawning, they will usually put their eggs on any vertical surface which includes the heaters, needless to say, you do not want that.
HTH