I never start our classes with any sort of stretching.  Some of my training partners do, that that's all good with me.  To each their own.  However, a recent report in the New York times (and several others recently) talk extensively about how static stretching (the kind we are all used to from playing sports growing up) is actually harmful.

It seems that a dynamic warm up like jogging, jumping jacks, hindu pushups, bodyweight squats, spiderman walks, etc, etc are the way to go.  My personal favorite these days is to do the 6 exercises Paul Schreiner showed at his recent seminar.

You can read the full NYT article here.