Learned this the hard way with chicken wings last month. Tossed them in cold thinking I was saving time, and the outside got nice and brown while the inside was still pink. Had to put them back in for another eight minutes, so I saved zero time and ate late. My sister just laughed at me because she always waits for the beep. Now I preheat for anything raw but leftovers like pizza or mac and cheese go in cold, no problem. That grandma pie story is basically the same lesson with more tears.