If you're worried about flat spots on your tires, you don't have to be. Unless you live in an area with extremely cold weather, your car will be just fine if you don't move it for a few days. Of course, if you know that a prolonged period of bad weather is coming, it's always best to err on the side of caution and move your car once a day or so. But as long as you're not parking on snow or ice, you shouldn't have anything to worry about.