As you know setting the caster is done between the axle pads and the bottom of the leaf springs where they set on the pads. Nothing says the bottom of the leaf springs are or must be neutral, leaning forward, or leaning back. However the springs are doesn't matter because the axle is ultimately adjusted as necessary with the wedges to set the caster angle relative to the ground.
So I don't think you can predict which way the wedges should be installed without measuring the angles involved in each case. That said it might be that more often than not they're one way or another on a particular year, model, and outfitted truck but it all comes down to what is needed; it's a final fine adjustment after everything else is put together.
Grigg
