I checked some of the style guides in my library to see what they had to say about the preferred form of n.b/nb/N.B./NB, and this is what they had to say (sources are arranged in chronological order, oldest to newest).
From Bergen Evans & Cornelia Evans, A Dictionary of Contemporary American Usage (1957):
n.b. This is an abbreviation of the Latin words nota bene and means "note well."
From Wilson Follett, Modern American Usage: A Guide (1966):
abbreviations. 1. The modern tendency in scholarship as well as popular works is to replace the Latin abbreviations by English ones; for example, ... note for N.B.
From Words into Type, third edition (1974):
The following Latin abbreviations, including some seen only in older works, are not often appropriate to text except parenthetically but are useful in footnote material:
N.B. (nota bene), mark well
From Warriner's English Grammar and Composition, Fifth Course (1977):
N.B. note well (from the Latin nota bene); always italicized or underlined.
From [Merriam-]Webster's Standard American Style Manual (1985):
Latin Words and Phrases
25. Words and phrases derived from Latin are commonly abbreviated in contexts where readers can reasonably be expected to recognize them. They are punctuated, not capitalized, and usually not italicized.
[Examples:] etc. | i.e. | e.g. | viz. | eta al. | pro tem.
Although this style guide omits any direct mention of nota bene, it seems fair to infer from the prescriptive and categorical approach to the topic of Latin abbreviations that it would endorse the form n.b.
From Stuart Miller, Concise Dictionary of Acronyms and Initialisms (1988):
n.b. Nota bene (Latin: "note well")
From Robert Hendrickson, The Facts on File Dictionary of Word and Phrase Origins, second edition (1997):
N.B. This is the abbreviation used for the Latin nota bene, "note well, take notice," often used in literary and scholarly works. Voltaire told of how a commentator on Lucretius by the name of Creech noted on his manuscript: "N.B. Must hang myself when I have finished." According to Voltaire, "He kept his word, that he might have the pleasure [of committing suicide] like Lucretius. Had he written upon Ovid, he would have lived longer."
From Joseph Gibaldi, MLA Style Manual and Guide to Scholarly Publishing, second edition (1998):
NB take notice (from Lat. nota bene; always capitalized)
From The Oxford Dictionary for Writers and Editors (2000):
NB New Brunswick, North Britain, (Lat.) nota bene (mark well)
According to this Oxford style guide, "n.b." stands for "no ball" in cricket.
From Martin Manser, The Facts on File Dictionary of Foreign words and Phrases (2002):
nota bene LATIN {mark well} verb phrase note well, observe particularly. ~abbreviated forms n.b., N.B.
From Andrea Lunsford, The St. Martin's Handbook, fifth edition (2003):
Latin abbreviations
In general, avoid these abbreviations except when citing sources:
...
N.B. note well (nota bene)
From Bryan Garner, Garner's Modern American Usage, second edition (2003):
N.B. is the abbreviation for the Latin nota bene (= note well; take notice).
And from The Chicago Manual of Style, sixteenth edition (2010):
NB, n.b. nota bene, take careful note (capitals are illogical but often used for emphasis)
The raw vote totals from these reference guides (counting both options offered by CMoS, sixteenth edition and by Manser) is as follows:
- nb 0 votes
- n.b. 4 votes (plus one prescriptive guideline that would seem to require it), ranging from 1957 to 2010
- N.B. 7 votes (including one specifying that it should always be italicized or underline), ranging from 1966 to 2003
- NB 3 votes, ranging from 1998 to 2010
Opposition to italicizing the Latin abbreviations thus seems to have begun at an early date, with little counterargument. Both the n.b. and N.B. forms of that abbreviation have had their adherents among style guides across the years, without pressing the rival punctuated form to the periphery. And NB has emerged relatively late—an unsurprising development, given the more general trend toward punctuationless abbreviations in recent years. Perhaps the more forward-looking question would be, By what date are i.e. and e.g. likely to lose their periods in standard usage?
