Cliff is right about burr formation because you are pushing metal away from the apex of the edge. As for heat, thats more complicated. The heat is more a factor of the right grit type and amount of force. Biggest mistakes people make are starting with too fine a grit and pushing the blade too hard to remove metal, which creates a lot of heat. A coarse grit will remove more metal with little pressure and little heat regardless of which orientation the edge is on. All I need is a couple of passes with a coarse grit to set an edge angle. Then refine with higher grits. The secret to not heating up is to not old the edge on the belt longer than an easy steady pass. The risk of injury or damage is higher with edge up orientation.
To each his own.