@evan [Cool URIs don't change] don't change.
Given how uncool so many URIs have proven to be, I'd want any additional instructions to further the "don't change" mission. Hmm, wonder what those would be, interesting challenge!
Is there any reason to think a URI having an inbox or being subscribable would be more likely to not change -- or more likey to change?
I do think a similar doc, perhaps w3.org/Provider/Style/Social would be...cool!