I wrote my first 'arc' by the seat of my pants. With the second arc that I'm writing, I've picked up an editor. I don't generally make spelling errors or grammatical errors, but there are some sentences/phrases that may jar the readers. My editor is from England, while I'm in the United States, and since I have as many European readers as I do North American readers, he really helps point out some of the stuff that NAers might understand, but Europeans wouldn't.
I don't write a clean copy the first time around by any means, so I try to make each chapter as perfect as can be before I publish it, because to me, this is art, and I'm presenting it in a way that wouldn't be as great if it wasn't as perfect as I could make it.
However, nothing can ever be -perfect-, and I'm sure in a year from now I'll look back on arc 2 and think it's in serious need of a re-edit.
Likewise, I know many very talented authors that use web fiction as sort of a 'first-draft', where they publish works for reader response/critique, and use what they're given for a solid version later on down the road.
Ultimately, it's really what you're intending to get out of it.