Your main task is to write, because without the writing, there will be no book. So for me, I generally write as long as I am feeling inspired, and rewrite, proof and edit mainly when I am at a standstill in my writing. This way I don't lose forward momentum when I have it. To put it another way, it makes for more efficiency, because I only "switch gears" when already stopped (so to speak). I've found that when I'm blocked, or not moving forward as fast, it's often because my overall sense of the book has subtly shifted, and switching to rewriting at those points can help me get back on track.
Of course, if you put off rewriting too long, you might end up writing a great deal of material that you don't end up using. But that isn't necessarily a bad thing, it can still inform the material that actually makes it to the page. For you, at this particular point, if you do feel an actual need (or desire) to rewrite, I would do it, but if not, I would push on ahead.
In terms of actual proof-reading, you'll want to make sure your book is impeccable before publishing. Unless you are an excellent proof-reader, you'll probably want to hire someone to do it for you, as it is very difficult to proof your own copy (although modern tools help).