I really struggled with that when painting mine. I think at the end of it all I came to the conclusion that painting the diff all black, or natural is generally considered correct because every car was a little different. Like data says, some cars just a dusting while upside down in assembly, others got more paint. So I don't think there is a fine line with how much.
But, if you go with the bare look, then in theory you should probably see the welding color marks & in reality see some "rust" which nobody is going to put on there suspension parts, so with that in mind they all end up over restored when they are clean as can be.
I have some pictures saved from a restoration shop that wend all out with natural steel, weld discoloration & partial black paint. Looks really cool, although I think the restorer went a little light with the black & had a bit to much natural metal showing on the parts.
I think thats why either way flys in judging. Could be a lot of black, or very little. So for me it came down to those rules & what did I want it to look like. I anguished for several weeks over this. I was very close to black backing plates, natural tubes, cover & cast differential housing. Several guys here did that & it looks nice.
I ended up going solid black so it didn't stand out too much against my frame & other solid black suspension parts.