As a former Radiator Shop owner I find the term "Winterizing" a little misleading. Basically if the cooling system is properly maintained, IT IS "Winterized"!
Your Cooling system is under the harshest conditions during the Summer. With the proper amount of antifreeze the boiling point of your radiator fluid is over 40 degrees more from boiling over than 100% water. With the added pressurized radiator cap giving the mixture another 3 more degrees per added pound of pressure, the boiling point can be raised to over 300 degrees before it starts to boil!
The main aspect of Winterizing your cooling system is the usual added 50% of antifreeze most vehicles have. That will protect from freezing and expanding in your engine should the ambient temperatures drop as low as 10 degrees below Fahrenheit. Not many places a Southern Californian will find that cold. A Minnesotan in 30 below will have the 100% antifreeze in their vehicle. It will protect all the way to 60 below. Way too cold to think about.