ServiceTechGuy: It is recommended to rotate them every oil change, and replace them if the tread gets below safety requirements set forth by your state, in Texas it is required to change the tires if the tread is below 4/32 inches on the front steering axle and 2/32 inches on the rear tires, also check to see if there is any odd wear on the tires as well as any bubbles or cord (wire) showing anywhere on the tires, also you can judge them by the mileage on the tire, most average people put less than 50,000 miles a year on thier car and tires and most tires are rated for 80,000 miles so you should be fine if you fall below those general guidelines, but check your tires carefully for any of the items i listed previously because if any of them are present then you are chancing a blowout. i hope this helps you and others, good luck.