I was taught it in a C152 but in a clean configuration, for the past 2 years though I have mainly flown an Archer III.
I think its definitely something worth demonstrating as part of PPL even if it isnt part of syllabus and I was probably lucky in the fact that the end of every lesson I had my instructor would have a quick 'playtime' where he would show you something not normally covered (I was also lucky enough to have the same instructor all through my PPL).
Mark 1, I have only used to drop through holes in clouds before I got my IR. As you say certain instruments are prone to erroneous indications and dont think I would like to try it blind! Final descent is another place although I have heard some guys I fly with say that their instructors on club checks have had a go at them for initiating a slip at this point.