Last sentence first -- no. I knew from the beginning that Pluto would have the last word and even how that would work out. But the actual wording didn't come until I was writing the scene and decided to end the story right there.
Incidentally, my editor at Scribner wanted me to change that ending; she called me four or five times trying to talk me out of it. But since it was the ending so many different threads in the story led to, I just dug my heels in and said no.