I suppose you do 4 man fireball? in that case the most difficult ones are 2nd and 4th and other even numbered ones.

I much prefer 3 man fireball, that way you can dump your CDs on Twin to advance faster or dump it on cases where your healers get conflag'd. It is even possible for one healer to get conflag and the other to get FB so both need to be in. then the MT also needs to be in. you need to get them out ASAP.