Placement of brewers belt...

I've been brewing for about a year now and I just found myself wondering today...Does placement of my brewing (heat) belt matter? I usually wrap it around approx. the middle of the the batch (on the fermenter of course). My train of thought being that the heat will radiate through the wort.
I started thinking today though that heat does rise so maybe I should be placing it closer to the bottom? What do you guys think?

From a winemakingtalk.com thread. Gotta love a good internet search.

..."I place mine at around the bottom of the carboy and use a rheostat to help control the temperature. While a carboy is a rather small container it is taller than it is wide so why risk creating mini Thermocline by placing you heat source at the top of the carboy."

Reading the instructions for use:

"position the belt around the outside of the pail so the belt is approximately 1/3 down from the top of the pail"