If you live in the South (which you do), the answer is yes. An overflow tank does a few good things. When your radiator gets hot it will burp coolant into the tank and then when it cools, all that coolant will go right back to the radiator. Prevents an air pocket and reduces air bubbles (which you really don't want...fluid does a much better job of transferring heat), gives you a little more coolant in the radiator (which is good--more is better) and makes it really easy to see the level of coolant and to safely and easily add more if needed.
