You always want to check the toe in in relation to the wheel hubs like Ralph said, don't check it at the tires.

Tires typically wear on the outboard side and rotating them is a normal procedure to maximize the wear and life of a tire. The outside wears because the is the part that touches on the landing as the wheels spin up. As the plane gets heavier the landing gear spreads out and the wheels go from a positive camber to approximately zero. The picture is for cars but it give you an idea of what camber is.

camber.gif