The pseudonymous alt is the way to go for this, unless you're mostly using twitter for things related to work. I bet some of your colleagues have alts :)

This is why I purge my old tweets automatically.

Nothing good has ever come from someone digging up old edgy tweets.

archive.org scoops up all kinds of tweets - deleting them isn't really a safe way to get rid of it, just fyi

Deleting edgy stuff at least gives plausible deniability.

"I don't think like that any more, that's why I deleted the tweet" is a good defence IMO.

Do those archive let you search for tweets?

sort of - if you search the wayback machine for something like http://twitter.com/username/* - it will list all the tweets it has - and then a user can download everything and search locally. so, it's not as simple as searching by keyword, but it might be something to be aware about, if you are concerned for that kind of thing