when you initially add the water, if you arent stirring well, the top will be warmer than the bottom. but if you were to stir until the whole mash was a uniform temp, the top of the mash would be the first place to eventually loose heat.
a mash heater isnt really ever *required*, but it makes life much easier if you are using a non-insulated container or doing large batches. it just takes one more variable out of the recipe.
if you are not recirculating, the best place for the probe would probably be dead center of the mash. this would give you the best average picture of what the temp actually was. but that could make stirring harder, etc.