I would like to nominate for "teaching aphorism most in need of jettison" right up there with "don't smile until Christmas" :
"Never write anything on the board that is not true"
I heard it first when i was in my masters program (I think) and i heard it most recently today out of a student teacher. It makes me want to hurl. Seriously this strikes me as something some know-nothing said one time that was subsequently repeated by know-littles who want to sound like they know what they are talking about.
I suppose I get why you'd want to adopt this as a guideline when you are super-green and can't manage the most basic of lessons but make no mistake, this is not how learning works.
If you are doing it right your kids will be making conjectures. Conjectures are guesses. Guesses are often wrong. Write them on the board. Be noncommittal as to your agreement with the conjectures. Give the children the air and light to disagree with the conjectures and argue with them. THAT'S MATH. BEING RIGHT ALL THE TIME IS NOT MATH. I CAN'T BELIEVE I HAVE TO SAY THIS.