How come you won't know what you'll do given you can pretty much see the party setup? The only thing you can't see is the TP bar of your partymates, in which case I have said that if they don't ask for it, why would you use it? You can spend your MP singing other songs like Foe Requiem and Mage's Ballad than Paeon.
The only time I'll let my partymates know what I will use is the time I'll use Battle Voice, other than that, regardless of you tell them that you'll sing X song or not, it won't matter.