That is true, especially with cars and trucks that have a full frame, but those rear axles on these unibody Stangs are off regardless.
I've seen the dealer aligning Mustangs and they always compensate the one side at the back from the other, before going to the front.

 

Here is the conplete list

055-102 $21.95 Prothane Bushings - Rear Tacking Arm


The panhard bar is at a 4 degree angle downward on
an OEM stock S197 stang.
It will shift the rear end to the drivers side under
rear suspension compression and to the passenger side
during rear suspension extention.
If it were level, then the suspension would always shift to
the passengers side during compression or extention
of the rear suspension components as the arc would travel
above and below the normal suspension at rest position.
As it is, it moves both ways. Thats the track bar adjustments
that are made during the NASCAR races thru a hole in the
rear fender umongst other adjustments....

Maybe this is an excuse to get a Watts link set-up...
No side to side shift with rear suspension compression/extention...