As Herbert pointed out and linked, that is the official policy, however as he also pointed out, it is widely ignored. I would venture to guess that almost all members don't even know that is the policy and, even those who do, don't use it much.
I believe that most of the US Census records used here as sources likely link to the page at familysearch.org that displays the transcription of the record or the page that displays an image of the record itself. The familysearch website actually gives us the citation they would like used - it's just copy/paste to get it and use it here, so I would expect that is what you'll see the most here. I also believe that it is considered adequate for use here.