Replace them when the cords show, damage requires a boot, cords break making bumps, or flat rate becomes unacceptable (this is rare for me on tires that rely on a protection layer to prevent punctures). The rubber has enough compliance that being squared off doesn't matter.

GP4000S with 1037 front miles + 5257 rear miles:


At that point I remove the front tire, move it to the rear if still good, and put a new tire on my front wheel because loosing traction on the front tire (perhaps because it deflates due to a blow out) is more likely to cause a crash than problems in back.

My current GP4000SII rear tire has 8578 front miles and 2862 rear miles. It still has lots of life left.