Huh???

at any teacher that says that. Letter bias is a myth and it can't possibly be based on anything, except a statistical analysis of every released test in a certain timeframe. Even then, it would only crop up due to random chance and not be a reliable indicator to alter test-taking behavior for the next test.
Additionally, you are looking at a random 1-in-5 chance at getting each question correct if you guess blindly. These are
independent events with no relation to one another. So it should not matter in the slightest if you stick to the same letter, or not. I guess maybe the test attempts to distribute A/B/C/D/E evenly for correct answers? I don't know if that is the case, though, and you could wind up screwed if you pick the wrong letter. The only real definite advantage I see to doing that, is it takes less thinking. It's only something you do if there is literally no time to do anything else and wrong answers aren't penalized. On the ACT I forget, but I don't think they are...maybe there is a compelling argument here for what you guys are saying, but I can't think of any.
Instead of any crazy rules like that, the only advice that should be given is to focus on maximizing your chances for success, which you can do by eliminating a definite wrong answer or two. Eliminate one and guess blindly, and your chances are improved greatly. Eliminate two and even further.