I just checked the temperature in my barn where I have several cars (West Michigan). Using a thermal gun I got readings of -15.F on the engines. This is getting close to my antifreeze protection level. As I understand, it is suppose to get even colder over the next few days. If you also live it the "tundra" zone it probably wouldn't hurt to recheck things. There could be a lot of downside if you don't.
Tom