I think it mostly depends on your process and a little bit of trial and error. On my system, I preheat the cooler with hot tap water (fill it with a few gallons and shake around while mash water heats up) and pour it out just before I mash. By doing this, I can use the formulas from Palmer (http://www.howtobrew.com/section3/chapter16-1.html
) to calculate the temp of the mash water. I'm usually shooting 14-15 degrees hotter than the mash temp.
I found that if I did not heat my mashtun before there were more swings and it was not so predictable. Another trick while you're dialing your system in is to try to miss high. To me, it is easier to cool things down (just add a little cold water and stir) than to heat up, especially once you get the grain in there.
Keep track of how you end up each time and compare -- you can get it dialed in in a few tries.
Hope this helps.