I have a question for you. How do you know when you should stop revising and start submitting? Or how do you know when you should start submitting a novel that you know still needs some revising?
I'm going to answer the last part of the question first. If you know the book still needs some revising, I wouldn't submit it. If the editor reads it in an imperfect form, they'll think that's the best you can do. You can't include a letter that says, "I know this still needs work and I intend to do it, but I thought I'd see if you liked it first." They can't decide whether or not they like it until they see it in as perfect a form as you are able to get it. So, if you know it's not done, don't send it in.
The first part of the question is a little trickier. The LDStorymakers joke that when you're ready to throw it under a bus/throw up if you have to look at it again/kill yourself rather than go through another edit, you're ready to submit. That's true, in spades, but not the entire answer. Some questions to ask yourself are:
1. Is this book as good as you can possibly make it? When you read it, do you find places where you think, "I could have done that better?"
2. Have you asked others to read it? Have at least three people, with critical inclinations, read it and picked it apart for you? Have you taken their advice (when needed) and done what they said?
3. Have you checked every spelling on every word, every punctuation mark, every page break, to make sure that you haven't left out anything important and that all your words are spelled correctly?
Unfortunately, there are a lot of manuscripts submitted that look like rough drafts. They are poorly spelled, badly punctuated, and do not look professional. Your manuscript needs to look crisp and clean. Your plot needs to be seamless. Your dialogue needs to be engaging and interesting. Your characters need to resonate with the reader. If any of these elements are out of place, you are not ready to submit.
On the other hand, perhaps all these things are in place. And if they are, why are you sitting here reading this blog? You should be at the post office! Go! Go!