You'll also end up being a faster driver if you practice first with bald tires...the car's limits will be extremely low and you'll be forced to learn how to handle your car at the threshold. After you put new tires on you'll be used to controlling your car at the limit, and have a nice buffer zone you can work your way up to.

I went to my first few autocrosses and track days with the balding stock tires...finished pretty far back in the pack. Everyone kept telling me to get new tires etc. but it was a good learning experience. I can't wait to go out with my new rubber

Shingo