Charles Nelson, in a comment here, reminded me of the fact that chemistry teachers also "lie" to their students. Thinking back to a science class I recently sat in on, the instructor said, "Later on you will find out this isn't exactly true, but for now..." At least he was sort of honest. Charles' example and the case of the instructor I observed are definitely examples of trying to make things simpler for the beginner.
So I go back to writing... Is it helpful for beginners to be taught these "rules"? I was talking recently with a friend who told me she still tells her students they can't start sentences with "but". I remember being taught that 40 years ago or more. But of course, you don't write much without starting a few sentences with "but".
My philosophy for the last several years has been to let students write. Period. I try to teach about audience, and that is maybe where I would tell them that some people will not react favorably to certain constructions like contractions or more informal language. But I am not even sure that I am not lying to them still. Will a philosophy professor react negatively to a paper in which the student uses lots of informal language if he expresses himself clearly? I don't know. Guess I should ask one!
Charles has a blog if you would like to check it out.