In my opinion, one should add them if one intends to put the car on the road or track. Trailer queens, maybe not so much.
What I seem to remember from back in the day is movement at the welded/leaded line where the quarter panel joins the roof. In some cases the paint would actually flake off at the joint, and there'd be a noticeable bulge.
The cars weren't all that fast back then, and it seemed to be mostly bigblock cars.
