I've read that these things were wrapped tightly enough to create metal-to-metal "welds" around the corners (which is why they used square posts, not round ones) which function much better than solder connections.
But I have seen them get rather dirty and corroded around the visible spaces between the wires, and long suspected that some degree of corrosion could work its way into those "welds". Therefore, I always blast all of them with contact cleaner, and on the most critical gear, with DeOxit. It does seem to improve things, and looking at them with low-power microscopes, they do get cleaner.
Now I have conflicting advice: 1. Don't solder them, as you can adversely affect the intrinsically better "weld". 2. Solder them, to create a bigger/better contact.
Which is right? I'd really like to hear a specialist metallurgist's answer, based on actual research. I'm tempted to look for some kind of conducting oily-silver solution to apply to them, to "fill the gaps"... or maybe I'll start soldering them, after really thorough cleaning.
Hhhhmmm....
OH, but I WON'T solder over the ones in my Yammie M2; all those wires and posts are silver (not copper), and silver oxide is a decent conductor, so as black as they get, they still perform well, better than solder, actually!