When I edit, I create a stylesheet specific to that book/author. Your own style of writing may be more suited to using the Oxford comma or avoiding it. If you have a character with the last name 'Gray', you might want to spell the colour as 'grey'. Maybe you want to italicize all foreign terms or maybe your book would have so many that you need to distinguish the commonly known ones from the obscure ones. Default to a major style guide when you're unsure what will read well, but where you have multiple options, choose what best suits your own writing.
I set up an Excel spreadsheet with the options I most often have to choose from. Here's my shortlist:
style base: Canadian English, US English, UK English
guide: whether you default to CMoS or other when your guide has no standard for something
numbers: formatting large numbers (1,234,567), what numbers get spelled out (one...ten, 11-19, twenty, hundred, 101…), how to note ranges (10–50 en-dash), do you write them out in dialogue?
decades: long: 1970s, 1970’s short: 70s, ’70’s, the seventies
seasons: capitalized or not
date format: July 1994; August 27th, 1927
centuries: Nineteenth Century, nineteenth century; whether it's hyphenated as an adjective
times: 10 am, 10 a.m., ten a.m., etc.
money: $25, $25.00, 25 dollars, twenty-five dollars
possessives: s’ or s’s
directions: capitalized or not, either as direction or as a place (the North)
celestial: Sun, Moon, Earth capitalized or not
wars: World War One, World War I, World War 1, First World War
provinces and countries: BC, PEI, US, UK or B.C.. P.E.I., U.S., U.K.
measurement: when Imperial and metric get used
address formats: are 'Road' and the like spelled out or abbreviated?
degrees: MD or M.D.
military abbreviations or ranks spelled out
Punctuation:
Oxford comma or not
Em-dash padded or not (spaces on either side)
Ellipses being font based (…) as opposed to three periods (...), whether a sentence ending with one is followed by a period
When single and double quotes get used and whether each go inside or outside punctuation
Formatting for foreign-language terms, legal cases, ship names, book titles, movie titles, painting titles, song titles, article titles, signs, nicknames, and slang terms (italics or Roman, single or double quotes)
Text styles for letters, emails, text messages, poetry, song lyrics, signs, etc.
Then there are common terms with multiple options, like 'grey/gray' above. Is superceded with a 'c' or an 's'? Is skeptic with a 'k' or a 'c'? Is percent one word or two? Are cooperate and stepbrother hyphenated? Do régime, résumé, and façade take accents?
Finally, I list recurring terms that might get misspelled and any proper names (people, places, companies, etc.).