[Dean]

Typically, cars have 1-2 degrees total negative camber adding the front and rear on one side. that makes up for the load/wear on the outside shoulders when turning. The harder you drive in the corners, the more negative camber you can use and still wear fairly evenly. Rotating tires is of course key as well. Rears almost always wear slower in general, and less on the outer edge than the fronts.

So on Imprezas pre '07 or whenever it was they redid the rear suspension. Rear camber was -1 degrees which wears the inner edge and the front was 0 which wore the outside and -3.35 caster all as I recall.