If you go second and have Yogg in your hand.. .Keep it. If you have 2 low cost spells and a Pyro... Keep it.  It is all just for fun anyway

UPDATE:  I yanked a Mirror Image and put in Evocation after playing around with it... I had a Sorcerer's Apprentice on board and dropped the Evocation and got a handful of Arcane Missiles and Pyroblasts.  getting 3 Pyros in a turn was pretty funny.