I don't know the specifics about the NSA (or about any other agency), but the whole "don't care much about past, caring about trust and transparency" is pretty standard security model in western governmental agencies. Notice that CURRENT illegal activities can be a source of blackmail - if I'd work for the NSA and run over someone with my car I wouldn't expect a three letter agency to come and make the charges disappear (and that's a good thing of course, we don't want an above-the-law military class).
As I said, I don't know the specifics of the NSA (maybe they do or do not care about future light drug use) but I would expect them not wanting you to continue any illegal activities, to a reasonable degree (continue stealing the office toilet paper if it gives you kicks).
Unless you're some sort of a world-level genius they would probably also shy away from evident extreme sociopathic behavior. From a reasonable human resources angle, not just security. Most of us wouldn't want to work with someone who released a worm that specifically targets thesis drafts for example.
ANOTHER EDIT: I really think the #1 concern for applicable defcon recruits is "computer crimes", not drug use. Found/tested exploits? Use bittorrent? Cracked WinRAR (and hopefully something else that is more than simply changing a je to jne)? don't be afraid to send a CV and be honest. That is all.