I don't really mind the tweet/rating request, whilst it's obviously to help Amazon gain more sales, it also helps get the author more sales if people highly rate a book and spread the word. Nothing wrong with a gentle reminder when you're done
Still, I think the original poster should have the option to turn off the reminder if they don't want it. Really the "share" option shouldn't even be displayed unless you've linked a twitter or facebook account to your kindle, or shown at most one time to let you know the feature exists and an option to disable it there and then.