If you're writing your book, even if you're FINISHED with it....even if it's like, 800,000 words long (and seriously, if it's that long you are sooooooooooooo not finished)- back to the point- if you're at that point, and it suddenly occurs to you that something might not be quite right, or you realize that you could make the book a zillion times better but it would require a major re-write....
DO IT ANYWAY.
DO NOT tell yourself that the book's "good enough."
Because if you do, you'll end up with a request for a full from an agent you're in love with, and you'll spend the next four days frantically re-writing around the clock (cracked out on cheesesticks and cherry coke zero) because you won't be able to get that re-write idea out of your head and you won't want the agent to read anything until you get it re-written.
I'm not even sure that last paragraph makes sense, but I can't even think clearly anymore. I need sleep and a shower.