For each boss there are different mechanics, different jobs that us Mages have to do. It also changes due to raid compilations and such. Weather your group has a hunter for Mag-maw can be the difference in you needing to go frost for Mag-maw or switch to fire. For the most part this is a GENERAL guide on what spec to go with each Cataclysm boss and varies.
Black Wing Descent
For Mag-maw most groups would prefer you to be a frost mage due to slows and such and the fact that your Aoe is mana efficient and mage armor helps you from going oom after the first set of parasites. With that being said, any Cookie cutter Frost Aoe spec would work just fine here.
With omnitron-defense you should have no problem going your highest dps spec, which Atm is Fire. Any other cookie cutter fire spec as explained in my other posts should work just fine.
For Maloriak it depends on gear and how your raid is approaching the boss fights and with what strategy. In most cases you should go fire, also on the frost phases you should volunteer to scorch the Ice tomb and have a druid healer to put a hot on them so they don't die. On green phase use blast wave, followed by a living Bomb on Maloriak and spread with fire blast.
For Chimaeron the same applys as Atramedes and Omnitron, best dps spec will work great, no adds here.
For Nefarian I cannot personally say what spec is best as I have not yet cleared this boss, but fire spec should be the best choice.
I WILL UPDATE WITH BOT/TOFW IN A FEW DAYS