I've always stuck to Disallow and followed -
"This is currently a bit awkward, as there is no "Allow" field. The easy way is to put all files to be disallowed into a separate directory, say "stuff", and leave the one file in the level above this directory:"
http://www.robotstxt.org/robotstxt.html
From https://developers.google.com/webmasters/control-crawl-index/docs/robots_txt this seems contradictory
| /*
| equivalent to / | equivalent to / | Equivalent to "/" -- the trailing wildcard is ignored. |
I think this post will be very useful for you - http://moz.com/community/q/allow-or-disallow-first-in-robots-txt