Hi Vincent,
During use, that it normally the place where the tablet gets really warm. It is the place where the heat-sink comes closes to the back-plate. Also the CPU is close to there. Most of the other components, like the voltage regulators, etc are also located on that side. So during charging it would be logical that the heat would come from there.
Although, if you only charge the tablet and you are not using it, then it should not get uncomfortably hot. I would recommend the heat mod at all times.