I brewed last weekend and used my standard procedure of getting the strike water in the kettle just right (166 F). I poured it into my mashing cooler (no pre-heating) and added the grain. I used a fast reading digital thermometer right after mash in and sure enough the readings varied by as much a 6 degree F, even after stirring. I left it alone and 15 minutes later the mash had settled exactly where I wanted it to be -- 151 F. At 75 min it only dropped to 150.
I want this batch to attenuate down quite a bit, thus I'm mashing on the low end. I'm using WLP007, aiming for a fairly dry, light colored PA. Three days into fermentation it looks fine. BTW, I used some Fermcap since I was using a 6 gallon better bottle and was concerned about head space. My larger carboys were tied up. Krausen make it to the airlock anyway, but I've seen worse. FWIW I mashed 12 lbs of grain in 15 qts of water using my 5 gallon round cooler (about max for that unit). OG was 1.066.