Finished or still Tinkering?

Do you know when you've finished writing a book? I'm not sure I do.
If I've reached the word count I wanted and the story has reached a grand denouement (Ha!) it's tempting to think 'Good that's done' and go on to something else. But...and it's a big but that won't go away... ideas start occurring in the little grey cells.

Thoughts like ... Didn't I intend to re-write chapter 4 in order to have the main character meet Meg then instead of two weeks later? Do I really need that prologue, or should I ditch it? What if I changed the POV and took out some of the minor characters to make the main story line more powerful? and the worst one of all: Good grief! I forgot to write the scene where she told the Dowager who had stolen the child!

Once this sort of mind set starts up there's no escape. Revisions are not only recommended, but required! This must be where it is bliss to have an agent or an editor read your work and tell you where you've gone wrong or where the story could be improved. The cold, clear eye of the professional must be invaluable. On the other hand, does it then become a joint effort? Depends on the amount of professional input, perhaps, but right now I'd settle for any at all!

Once you've written all the bits you forgot to include in the first draft, re-written the paragraphs to match where the plot changed as you went along, tweaked characters who probably said and did the wrong thing at the wrong time, then there's the endless fiddling with the English language. There are so many ways of saying the same thing, so many variations of word, of word order, where to place the comma or where to use a semi colon that this sort of re-writing can go on forever. Once I had a good grasp of where to use a comma, but five years of critique group work has left me confused because Americans do it differently to the UK. Their use of language is definitely more fluid that ours and I cannot help cringing when I see nouns used as verbs, or verbs used as nouns. I passed an example last night: "the crystal fell in shatters to the floor." Ah well, some readers will love it.
When you've gone through all these possibilities at least once, then it might be possible you novel is finished. What do you think? Another run through, just to be sure?