Honestly it depends on your party and the boss in question. If you use Shroud of Saints as soon as you are missing around 800-1000 MP and continue using it every time it is up your MP lasts quite a lot longer. If you have a BRD in the party then MP shouldn't be an issue at all in any situation and you can just go nuts.

For some bosses, like the boss of Aurum Vale and the flan boss in Wanderer's Palace, WHM tossing out a Cleric Stance Holy or two is a huge advantage despite the MP cost.

That said, if you are in a party where people aren't as geared and fights take longer, you will definitely have to be judicious for bosses. During most trash pulls though you are perfectly fine nuking if your tank is geared appropriately.