From my experience, the oscillatory pitching effect of the PA28 at the stall is a result of the control column being held and maintained fully back and is a product of the high level of pitch stability enjoyed by that aircraft. If you get to that stage with a student, he/she is flying the aircraft to the limit of its angle of attack capabilities and is in a full deep stall. Remember that the aim of stall training is to recognise the stall at it's onset and recover with the minimum height loss. If you ever take the student to this stage, it should be once and for demonstration purposes only. The aim of this exercise is to recognise the warnings of an approaching stall and avoid them in the future. there should few reasons for the student to take a PA28 to the 'nodding' stage.
I'm glad that someone has bought up the subject of glider stall training. I first trained as a glider instructor and IMHO, glider stall training is superior to powered. Glider pilots are more likely to fly a wider range of types than power pilots and at speeds much closer to the stall. My experience comes from the Bocian (stall below 500' and it's goodbye) through the tailess Fauvel 28 (hoik the stick back and you can flick loop) to the Dart 17 with no washout (stall and you're guarenteed an incipient spin). Some gliders have truly horrible stall characteristics, but most are benign.