I disagree Shefly. First, it is good to practice on flat pedals first just to get the motion down so you don't have to worry about unclipping etc. Of course, you should practice with clipless pedals before you go race on them! DUH

Also, I'm not sure if you've had this happen but I have; and that is when you get off, you unclip your left foot, but it slips off the pedal and that's always fun; and also sometimes I've had it reclip by accident and that is kind of fun too! So, that is the worry for me.