While I agree that fire will be better for movement/add fights, I don't think Deathwhisper would be such a case. Unless you're doing heroic mode, the adds die so fast that LB won't go off. Most guilds put ranged on the boss, so the only time you have to move is for DnD or shades.
This is what I'll most likely be going come 3.3.3:
Lord Mar: Toss up, Fire has dots to tick and pyros to throw during Bonestorm if he goes out of range, but that isn't too often.
Lady Deathwhisper: Arcane
Gunship: Fire, can you say LB spam?
Saurfang: Arcane
Festergut: Arcane
Rotface: Arcane
Putricide: Fire
BPQ: Fire, better range, and the fight does have some more movement in it.
BQL: Arcane
VRW: Arcane
Sindragosa: Arcane with IA if you have it. Fire might be attractive if you don't since it has a longer cast time which helps managing the debuff. Also, I've noticed that a FB/pyro combo only results in 1 stack instead of 2. This may be the result of my higher latency however.
Lich King: Fire, possibly FFB.