A shock works best if it is 90 degrees in relation to the arm/arm hingepin when looked at from the side. Anti squat kicks the rear arm up in the front, which would require the rear shock to be laid back some.

If you look at the stock shock angle created by the upper shock bushing, adding more shims to space the top of the shock away from the tower would decrease this angle (in relation to the arm/arm hingepin).

I know it doesnt seem like much but, it can decrease rear traction if it is angled back too much.