I'm with most of the people here in that I ride my tires till the threads show which means having flat treads for awhile. I wouldn't do that for racing though as the flat spot can cause a little squirminess when rolling into a turn and possibly losing traction in a hard corner. With the likelihood of having a pack of rabid racers behind you any slip or fall can be a disaster. Do yourself and your fellow racers right by using reasonably fresh tires.