Question: While playing a character with a spellshield (Nocturne or Sivir) do you need to shield the bomb's placement or the bomb's explosion?

Answer: 
Spell sheilds do not stop the actual explosion, however Zilean can't place a bomb on you if you have your spell sheild on.
As already said, every spellshield (nocs, Sivirs) shields the PLACEMENT, and so does banshees. The only things that shield the explosion/the dmg are Morgs Black Shield and Hexdrinkers Shield.