Here's my personal flowchart:
Are both healers alive, the situation is non-critical, and there are no mages in the party? Don't sing.
Mages in the party and it's a DPS race? Sing Requiem
Healer died? Sing Ballad
People keep dying forcing healer(s) to rez in the same battle? Why the hell are we still alive? Anyway, sing ballad. Usually only happens in ST.

Part of being a bard is knowing when to sing...unfortunately the line between "Singing would be helpful" and "it's a wipe" is RAZOR thin.