TL;DR - I almost never play a song without Battle Voice along with it. The numbers just don't make sense no matter how you twist the math.

The biggest thing with songs is that you need to make a lot of judgement calls.

4 man -

I don't play any song but FR. If I need to play Mana, something is very wrong. I will do it on request by the healer if the request seems legitimate.

8 man -

Mages Ballad -
I pop this + Battle voice if I see a healer get battle rezzed or low. Once battle voice is gone, I drop mages ballad. I use 1500 mana as my "gauge" for playing mana.

Armys Paeon -
I tend to throw this up rarely, but always with Battle Voice. I use it on request from the tank, or transition phases of fights where I know I won't be attacking due to whatever reason.

I tend to play FR in bursts, again, with Battle Voice. I drop it when BV is gone, I may need that mana for Ballad later.

Obviously things like Titan's heart phase, etc, you should throw FR+BV up.