Originally Posted by LowCel

Are you seeing any threads? If so toss them. Otherwise I would probably wait until I started having problems.

Originally Posted by merlinextraligh

The profile on a tire will square off, long before the useful life of the tire is up. The squared off profile isn't a problem, particularly on a rear tire.

If you're particularly risk adverse, you can take squared off tires off the front and put them on the back.

Personally, I run tires until the cord shows.