You don't always want to be panning, but should you try to freeze the car solid or do you try to keep a little motion?
You should always keep the wheels turning a little bit. Obviously there are exceptions, but it really depends on the car, the race, and the angle from which the car is coming at you. If the car is coming straight at you, you can use a higher shutter speed because you can't see the wheels. If you turn it just a little bit, though, so you can see sidewall of the tire, there are logos that will give away what shutter speed you're using. Anything below roughly 1/1000th and it will look frozen and sharp. If you can see the sidewall of the tire, it's going to have a big white or yellow logo on it. You want to get some motion on that.