I have heard Google and at least certain others respect a robots.txt file, and someone has suggested to me that Google will even use one as a reference for clearing disallowed things from its current index. So, something like this (edited for your own situation) might be a beginning point:
User-agent: *
Disallow: /wp-
Disallow: /feed
Disallow: /feed/
Disallow: /author
Disallow: /cgi-bin/
Disallow: /wp-admin/
Disallow: /comment/
Disallow: /comments/
Disallow: /trackback/
Disallow: /wp-content/
Disallow: /wp-includes/
Disallow: /wp-login.php
Disallow: /wp-content/cache/
Disallow: /wp-content/themes/
Disallow: /wp-content/plugins/
Go WordPress Dashboard
Settings >> Reading >>> and tick >>> Discourage search engines from indexing this site
blograzzi’s advice would be a bad idea, as it would stop your WHOLE SITE from being searched. So you’d never show up on Google. Ever.
So no, don’t do it unless you never want to be googled.
Keep in mind, blocking google from browsing uploads may result in weird search displays, as your images won’t show right.