Depends on what you mean by editorial work, but certainly at least good familiarity with AP and Chicago style guidelines and possibly Words Into Type. If a writer it may be less important, but if an editor, it's essential. Some hands-on experience with some kind of publication such as a college newspaper or something like that would help as well. Would it be more as a writer or an editor?
I did a lot of it full-time and still do freelance so if there's anything else with which I can help, PM me if you'd like.