I rarely notice buffs when I'm running heroic dungeons. Granted, every buff helps in maximizing DPS or survivability, but I personally don't find it really necessary. Do you absolutely have to have Blessings of Kings or a Prayer of Fortitude? I've had players who have absolutely refused to pull until they get all the buffs they want. I'm sitting there looking at their health and their gear and it isn't like they're fresh 80s or anything. We're talking tier 9 level stuff or higher. I don't think having Dalaran Brilliance is going to shave off a significant amount of time in a run personally.
Now when it comes to raiding, I make sure I have everything. On progression raids, I always check to make sure every buff that is available is active. At the same time, I'm a little more relaxed about it when taking on trash. I don't drop a Fish Feast for the trash leading to Lord Marrowgar, for example. I wait until we get to the boss itself before laying out one of these.
What? The feasts are expensive!
Anyway, what about you guys? Do you require every buff before tackling anything in a heroic? Are there certain buffs you can live without during raids?