Sometimes. I have heard that flat insulation is helpful only for about 4' from the edges, waste to put it out in the middle, this is with the footings insulated as well. If you are trying to get maximum thermal mass you also want to heat some of the dirt below the slab. The heat can only go back up if contained on the sides. Takes longer to heat it up initially but it also takes a long time to cool down. I will try to capture some heat from my forge exhaust to help keep the slab warm. Also to heat my slab will be solar panels and a wood stove with a boiler unit.
You need to read the info on the link I sent, there are others as well with good info on websites but I think that one is best/most info.
Here it is again: http://www.radiantdesigninstitute.com/index.html