Okay, so you're writing a novel, book, literary piece, short story, etc. Typically that means you have something to say. Maybe you're not the best speller or grammarian in the world, but you have the right to tell your stories, just like anyone else.
So is the story more important than the spelling and grammar?
Yes, and no.
When you're writing your story, you're weaving a tale that, most likely, you will want read by more than just one or two people. Yes, yes, of course you're writing your story for you, but be honest with yourself: wouldn't you also like for other people to enjoy the story as much as you have?
If so, you're going to need to get a copy edit.
You want to present a polished manuscript to your publisher. It doesn't have to be perfect (no manuscript is), but you do want it to be as good as it can get so that the publisher doesn't take one look at it and say, "this is a mess, forget it."
There is an exception when it comes to spelling and grammar, though, and that's in your dialog. When characters (and people) speak to one another, they don't always follow the rules of grammar (they split infinitives, just as I did here, and they even do more than that) and the spelling may reflect their accents. They say, "I could care less," instead of the correct "I couldn't care less," and they say, "ain't," "carryin'" and "Australopithecus afarensis."
Just wanted to see if you were still paying attention.
So, with the exception of dialog, you want your narrative to be spelled correctly and use relatively proper grammar. Don't make it a mess. Turn on that spell check, and have a couple of people go through and edit your copy once you're finished.
Remember, even editors, when they go through your manuscript and make changes, are capable of spelling and grammar errors. No one is exempt. Always check your work.
Sometimes, it's the difference between having your work read and having it trashed.
