When teaching, it’s easy to fall into having a mantra, simply because you repeat things over (and over and over) again. Seriously. It’s like having 25 toddlers at once. (There are always a few kids in a class of 30 who are on their A game.) I’m sure you’ve seen the memes.

In terms of writing, my mantra was and still is: Your job as a writer is to communicate to the reader.
Sounds obvious, right? But I don’t know how many times I wrote “Errors undermine your message” on student papers. I’m talking about careless errors, egregious errors, like misspelling your last name or complete failure to use capital letters.

The same mantra holds true for us as writers. It’s our job to communicate clearly, and grammar/mechanics, as much as we may hate them, are part of that game. (Okay, maybe hate is a strong word. How about dislike. Intensely.)
Writing is hard. There’s so many proverbial plates to spin at once that grammar can easily fall to the wayside under “I’ll fix it later.” And for most things, that’s true. Your internal editor shouldn’t keep you from getting words onto electronic paper, but at the same time, your writing shouldn’t be sloppy or lazy because editing will be so overwhelming that you’ll balk at attempting to clean a piece. Or worse, you’ll do a half-hearted attempt at them.
And the fact of the matter is that works which have a lot of errors take multiple passes to catch them all. Are you willing to make many, many editing passes of a 70,000 word novel? Willing to pay an editor to make many passes? Or might it just be simpler to write the cleanest you can from the beginning?
I’m not talking perfect, shiny, spotless drafts, just good ones. They really do make life easier in the long run. I keep a list of the errors I typically make and check (and recheck) for them. I typo “form” for “from.” Spellcheck usually doesn’t catch that. May/might and lay/lie are other issues I know I’m likely to get wrong. Often I avoid using them, but if I do, I consult the CMOS to be sure I’ve got it right.
When I taught Creative Writing, I frequently had aspiring young writers insist that catching grammar/mechanics errors was the job of the editor. I laughed really hard when they said that, and they’d stare at me with a shocked look on their faces.
“Yes,” I’d tell them, “it is the job of an editor to catch errors, but you’ll never get an editor with sloppy writing. Or a publisher. Or an agent.”
“Oh,” they’d say.
Then I’d tell them the other part of my mantra. “Whatever you hand a reader communicates more than words. If you don’t respect the reader enough to hand them a work as clean as you can make it, they know you don’t care about them. And they won’t want to read it.”
If you’re in the submitting trenches, sloppy writing is an easy reason for rejection. Agents and publishers have so many submissions that they’ll look for any reason to pass on a work. It’s not cruelty on their part. It’s just the way things work. And rejection sucks. So make sure the person you’re subbing to doesn’t get lost in careless errors. Let them get lost in your story instead.