There does come a point in time when you know a song is done, call it a feeling you get if you will. To over scrutinize a song after that point could lead to seriously overproducing the song or changing it so much from its original idea that it loses the magic it once had.

When I record or produce a song I get it to the point where I feel it is done and then try not to look back, at least not too much. The reasoning for that is we can always find flaws in our work but flaws are OK. Music isn't meant to be perfect and while we should strive to make our music the best we can, perfection isn't necessarily something we should achieve. Steely Dan is a good example of this attempt at achieving perfection. Donald Fagen is a stickler for perfection and if you listen to many of the newer Steely Dan albums you'll notice they are extremely sterile sounding. A fine example of great players, a great recording, and yet the end result has something missing because it is so perfect.