In the case of cold crashing, Palmer has told me that too rapid a reduction of temp will cause yeast cells to release lipids which can have a negative effect on foam. Being me, I ignored him and did it anyway. I didn't detect any problems. Sometimes reality astonishes theory.
Old topic I know, but I thought I would share my experience which has shown me that Palmer is correct. I heard Bamforth speak on the topic and he blathered on and on about how they crash cooled Carling Black Label...... blah blah blah...... but in the end he agreed that shocking the yeast releases proteolytic enzymes reducing foam. YMMV, but I believe slow crashing has a benefit but also the cost of time.