I have seen NARA both spelled out and abbreviated in Ancestry.com census citations:
(Note: items in bold for this question only, not actually bold in published citations.)
A) 1870 U.S. census, Neosho County, Kansas, population schedule, Ladore Township, pp. 14–15 (penned), dwelling 132, family 132, James Schafer [Shaffer]; image, Ancestry.com (http://www.ancestry.com : accessed 2 November 2018), citing National Archives microfilm publication M593, roll 440.
B) 1900 U.S. census, Stark County, Ohio, population schedule, Sugar Creek Township, enumeration district (ED) 146, Sheet 16A, Beach City Village, p. 267 (stamped), dwelling 338/340, family 345/349 (numbers corrected by enumerator), James Shaffer; image, Ancestry.com (http://www.ancestry.com : accessed 5 November 2018), citing NARA microfilm publication T623, roll 1323.
Is there a rule/guideline for using the abbreviation vs. spelling out?