I write in OpenOffice and have virtually all the autocorrect options turned off. It's actually good even for regular fic because very frequently what it gets corrected to isn't what one wants anyway. I have it default to UK spelling (rather that US English as I would otherwise) though since I only write HP. It's especially interesting when I find out certain words are American English that I really had no idea. So it just underlines my misspellings but doesn't fix them which is perfectly fine for FWF.