You could probably get arguments both ways, mostly based on theory or perception, real or imagined. The basic notion is that the shackles will swing sideways when steering action on a cross steer car is induced. Of course since that means the car will be turning there would also be sideways inertia too. Is it enough to cause a problem with handling? Most people probably can't tell, but some probably think they "feel" it. Either the dead perch or panhard bar will limit that, though the dead perch could limit the arc of one end of the spring end movement causing slight bind and a resultant harsher ride. The panhard bar swings in an arc so to the fixed length axle there could be some tugging and side to side movement induced. Neither of which is very significant, but to those who might want to "feel" something it could be perceived. Stock Fords started with cross steering in 1935 and went a fair number of years in the beam axle era without a front panhard bar. Dirt track cars are pretty much in a constant turn, and ride quality/feel/comfort isn't a priority so fixing the spring at one end may be fine for that situation.

As for retrofitting a panhard bar, you could probably make the frame attachment bracket a bolt on with minimal paint disturbance if you were so motivated.