In the May 4 Hot Links, there was a brief discussion about how important it was to score the first goal.  I remembered that King Kaufman, former sports columnist for Salon.com, addressed this issue in some detail a while back.  The short answer:  the first three goals are each important, to a similar, though not identical, degree.  But the stat-heads here might appreciate the two articles here, and here, for some interesting additional data.