For performance driving the worst wear areas of the tires are the outside edge on the front tires. With camber plates, you can even the wear out by forcing it to drive on the inside edge most of the time and not roll as hard onto the outer edge.
The rule for autox:
- If you can see cords, stop and replace the tires.
Once you autox a few times, you'll be able to tell when your tires are no longer holding any grip. Usually you'll end up replacing tires before you need to because you just want to get grippier tires.