Offset shackles

Where is it written one must ALSO move the front hangers in?
I believe in many cases the springs are angled in at the front. There are minor advantages to that angle being angled in. Most cars have tire clearance issues behind the axle, and offset rear shackles address that quickly, but the spring perches still must be relocated in any case. The rear offset shackle distortion mentioned is IMO without basis in any street driven car.