Attacking twice with "Black Luster Soldier - Envoy of the Beginning" can be tempting, it could deal a lot of damage to the opponent. If the opponent has a face-downmonster you should use the other effect and remove it from play. You may not be able to attack this turn, but you would be sorry if you attacked a face-down "Night Assailant"!
If the opponent has SetSpell or Trap Cards when you Special Summon "Chaos Emperor Dragon - Envoy of the End", it is often best to activate its effect right-away. If you wait, they could activate a Trap Card and destroy your monster before you make an attack. By activating its effect, you can clear out the whole field and deal a large amount of damage to your opponent.