I don't worry about "bald" much, since most of my tires don't start with much tread pattern. I might make an exception for a longer ride, perhaps over 200k (or overnight). Most of the time I toss a tire because either the casing threads are showing, or because the profile is really flat and I've had a spate of flats. When the softer rubber starts grabbing crap off the road and flatting, I figure that tire has done its duty and is ready to retire.