I don't know what you mean by bald. To me, "bald" is a car tire term referring to having no visible tread. But most bike tires are smooth so they are "bald" from the get go. Now I have worn all the rubber clean off tires with just the pale inner layer showing through. I have always taken that to mean that it is time to replace the tire before the next ride. I have had tires do this about 30 miles into a century and finished them ok. But it has been a long time since I have let things go this far. Usually, when the rubber gets thin now, I get a series of flats and that "encourages" me to put on a new tire.