I suspect you're over thinking this. I have yet to see a builder who uses 316SS fasteners exclusively.
Morris uses/used (when I have been there) 18/8 fasteners by StaFast and they get them from: Standard Fasteneings
in New Bedford, MA..
I have yet to see a box of 316 when I have been at the factory but I am going to guess they use them for some fittings.. Morris customers are pretty picky..... 18/8 good enough for a Morris.......
I have almost always used 18/8 but on occasion, where strength is not as important, I will use 316. I get all my fasteners from KL Jack a nut and bolt wholesaler in Maine.. Never had a corrosion or rusting issue.