Consistency counts for a lot, and can cover many writerly issues. This is not to say that if you’re a consistently bad speller, everything will be okay, because that’s not the case at all. And being a consistently bad storyteller will not put you on any best seller lists.
What I’m talking about here is perhaps better described as a “style sheet.” A style sheet is a set of notes for yourself – and perhaps for your editor, too – about how you are choosing to spell, punctuate, and capitalize certain things in your story.
Let’s say that you’re an American author, but you’re writing a book specifically for a European audience and you’re planning to submit to a British publisher. Because of this, you have made the personal decision to use British spellings: favour instead of favor, honour instead of honor, theatre instead of theater, etc. You’ll likely get a red underline in Word every time you type one of those words with an extra u, but for your purposes, it’s not a misspelling. Just make sure that you use those non-American spellings consistently throughout the entire work. Writing about the color of the honourable judge’s hair will just confuse everybody.
If you’re a fan of the Oxford comma (or serial comma), then make sure you use it in all instances that would warrant it. If you write “I ate beans, chicken, and corn bread,” then don’t neglect that second comma in a similarly-structured sentence later in the book. (Side note: I’m not advocating for or against the serial comma, or endorsing any particular style guide. Just pick your favorite and stick with it).
If you have a character named Sara at the beginning of the book, make sure that her name doesn’t change to Sarah halfway through. Or if the town is called Bellavista in the first chapter, it shouldn’t be Bella Vista in chapter two (unless the name change is part of the story).
Whether you’re adhering to the rules of a specific style guide or not, a personal style sheet can be a good idea to keep yourself consistent. An alert reader will notice inconsistencies – whether they’re big plot holes, or just little misspellings. But either way, inconsistency makes you come across as a sloppy or uncaring author, and no one wants that. So do yourself a favor, and make sure your writing is consistent.