I believe in the editing process. (Man, I'm obsessed recently with "The ____ Process" see; my blog, the last blog post, the title of my novel...)
It's hard because I want to re-edit pieces I've had published in Zephyr, or something else...something that I just didn't think of at the time. Right now my manuscript is floating off in space with agents I've submitted to...and while they're reading where I was at at that point...I'm continuing to edit. Why? Because I feel like no matter what, until the piece is published, it can always get better. Always. Even if you've submitted it for revision, you yourself should be revising it...or at least that's how I feel (I could be wrong).
I really enjoy editing with Laura (my sister). She goes through and says "This sounds awkward, I don't like this...I still don't like this. Take this out? This is redundant." I really, really appreciate her honesty with me, and though sometimes it hurts, I know she's doing it to help make the piece, MY PIECE, better, and that, my friends, is the end goal--having the strongest thing you have to offer.