Interesting post, Graham.
Your findings are very similar to my own.... When I was preparing for Model Rail Scotland, I did some very close and careful examination of the two power units on the class 170 to try and establish why they were running so badly over the two days of the Newbury show. I decided to do a deep clean of the wheelsets....and I found huge (in T terms) lumps of crap jammed in the dimples on the faces of the wheels and around the pivot plates.....exactly where Graham has indicated in his post.
Here's a very badly taken photo of the kitchen towel I used to do the cleaning on:
A wheelset can be made out with some fluff around the axle....but the important bits are all of the other black bits. All were levered out of the machined dimples in the outer faces of the wheels. This is one DMUs worth!!!
I followed this exercise up with doing the same deep clean on my two HST sets and three or four locos. Most had smaller lumps of crap!!
On day one of Model Rail Scotland, all power units ran without any power-loss related stutters. I repeated the cleaning exercise in between the days, which seemed a waste of time as there was little build-up to remove.....but a well worth exercise! I've never had so much good running before!
So...bottom line....yes, these are crucial electrical contacts....but I would suggest removing the axles and cleaning, then reassembling dry!