You are safe from LJ magically gaining the knowledge of your twitter account and tweeting every time you make an LJ post.

You are not safe from someone making a comment on a post on you LJ, and publishing a link to that comment on Facebook or Twitter. Even if the post in question is friendslocked, if they can comment on the post they can publish that link to Twitter/Facebook. I don't know how much information goes with the link, and I'd assume the link still won't allow unauthorised folk to read the whole thing, but it's definitely not ideal.