I recently saw an infusion mash tutorial that recommended stirring every 15 minutes to 'eliminate hotspots' and it puzzled me. I stir enough to equilize the temperature (and take temperatures at different locations in the mash to make sure) and then seal it up until the next temp change. There is no way an infusion mash is going to spontaneously generate hotspots! Of course, this is not true with a direct fire mash tun. I also think that if the temperature isn't even when your done with dough-in, stir more then, don't wait 15 minutes. You'd just be letting different enzymes do work in different parts of the mash, and that's not what you want.