It depends. Are you using a-arms or a solid axle or some form of rear swing arm? Typically you want the shock to pivot on the bolt through travel instead of the bushing or rod end. On a-arms you want the lower and upper shock parallel to the arm pivot bolts. With a solid axle (not always) most would mount the bolt on the axle end perpendicular to the axle tube and the upper would differ whether it uses links or leaves. With links the axle can move fore and aft through the range of travel more than left to right so you’d want it parallel to the axle.
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