I teach it in dancing, always have at least one foot forward. What I've been told by my mother who is an elementary school teacher is it delays or hurts the development of their knees and hips. It's a big no-no at my parents home, and at mine as well. All we say is feet in front please.

Here's two articles to read: http://www.pediatricservices.com/parents/pc-22.htm and http://www.physioquestions.com/2011/...-it-ok-or-not/