I watched this thread with some interest. I just purchased a 1989 PS34. Just before this thread started I had watched my broker back the boat in a straight line. I was surprised he could do it. He told me the key was to put the rudder all the way to starboard before putting it in reverse.
I finally closed and last weekend got to play with it. I found that it works as long as you use very low RPM. I found I could back in a straight line if I first put the rudder all the way to starboard, then put it in reverse at about 1200 RPM. If I gave it too much RPM, it walked to port substantially.
I have a two-blade fixed prop, so that might have an impact.