I usually cast it when ever I can. Why? Because you can't let your tank or your party members die. Especially the tank if they have stack on stack of crap on their party bar. So what will it be? Esuna the crap out of it until its gone and keep casting cure 1? Or stoneskin and then a bunch of cure 1?

I don't know your combo pattern but usually when I start to run out of mana almost half way, I use Shroud of Saints just in case crap is about to hit the fan you know? You can stone skin at the beginning before the boss fight and then just at the beginning when the tank taggs the boss a few seconds later. But as you keep fighting, the last thing you wanna think about is buffing in crisis times. You know when you have to run to get rid of your doom stack for example. Help to get rid of the temple bees etc etc.... But if everyone is in perfect health and your mana isnt stupid low then yes stoneskin.