If you look from the side of the heel, you can see the toplift. Follow the seam between the toplift and base to the rear outside border of the heel. When you are down to the thickness of a dime in that area, it is time for new heels, and you have avoided damage to the base. If you wait until you see the light-colored base at the rear outside borders of the heels, you are causing unnecessary damage to the base.
Soles
Do not wait until you see a hole. In many cases, if you turn the shoe upside down you will see wear in a circular shape. In other cases, you may not see the wear, but try pressing the center of the sole with your thumb. If you feel weakness or sponginess, it is time to replace the sole and you haven’t caused unnecessary damage to any other components of the shoe (such as welts, insoles, or sock liners).