Hmmm! That's a tough one. I hope not to bore you with platitudes, but I would say that really you ought to make the book the absolute best that you can. Or at least, you should not be rushing happily off to the post office thinking, "I think this is pretty good, but I know the middle is sagging terribly" or "the stepmother's voice is really not wicked enough and that's obscuring her motivations..." If you've made the voice and plot as strong as you can without outside guidance, the work should speak for itself.
But as you say, yes, I expect books to come in with flaws, and I'd be out of a job if they didn't! I've bought first novels that needed lots of work. But even if there's something not quite right going on with one or two of the book's elements, others (like a really unique voice, or a totally cool concept) will be so strong and so evident that I'm willing (or dying) to take a chance on an author.
So what's my answer: don't worry about making it perfect, just worry about making it darned good!