No, you are not required to know, let alone follow, these rules. That’s what copyeditors are for.
I disagree with this rather strongly.
First, I’m not talking about email or other informal media. I’m talking about writing for which you desire a professional appearance. That includes formal letters (on paper or electronic), business writing, web copy, anything meant for print publication, etc.
As to why you shouldn’t just leave it the copy editor:
1) For everything except print publication, you are unlikely to have a copy editor, and even for print publications copy editors are becoming increasingly scarce and overworked. Responsibility for producing copy that looks and reads right is increasingly falling on the writer.
2) If you are trying to get something into print, adherence generally recognized style rules in the first draft you submit will increase your chances of publication. Yes, few people know the difference between an en dash and em dash, but you can almost guarantee that the person deciding whether or not to publish your piece does. Of course, dash usage is highly unlikely to be the deciding factor, but a submission that shows the writer knows her craft elevates that submission in the eyes of the editor and tells her that this writer is a professional who has taken care in writing the piece and will more than likely be a pleasure to work with.
3) Copy editors have limited time with a text. Every minute spent nitpicking punctuation marks is time not spent on the more important aspects of copy editing, like improving the clarity of the text or suggesting alternative wording. And sending relatively correct copy to the copy editor, who is probably a different person than the editor passing judgment on the piece, starts your relationship with the copy editor off on the right foot.
4) You are the author of the text, which means despite what the editors do to it, you own it. You can’t exercise responsible ownership of a text if you don’t understand why the editors are making the changes.
5) Writing is a craft, and punctuation is one of the tools of that craft. A good craftsman knows his tools. If you don’t know the standard uses of basic punctuation marks, and the dash is a basic mark, you can’t use them to make your writing fully expressive of what you want to say.