I won't hurt to do the arrival time just pick as early a time as is available. If you get there earlier then your assigned time there are things to do in the waiting area like photos with Disney characters. If the check in area is not too busy I think they will let you check in right away. At that time they will give you a card with a group boarding number which will be called over the public announcement system. When your group number is called your may board the ship. I always do the online check in on the first day I am eligible and the highest number I've ever gotten was 7 and I boarded within a half hour of group 1.
Good Luck.