Usually requiem is your top priority song if there's at least one magical DD in your group. Requiem has the bonus of not lowering your own damage too. Paeon is useful if you have for example two BRDs and are doing big pulls which you'll AoE down. Swiftsong is great between pulls (I use it as soon as the last mob dies to save MP and make sure I'm not running to next group with requiem up...or aggro the colibris in longstop...). For ballad, it's usually a good idea to use it if the healer has died (especially if they're WHM and don't have shroud up) or has to raise many party members.

Ballad has its uses, especially when learning new content or running some difficult instances with solo healer. We use ballad regularly in turns 5 and 7, for example. It's just not needed for content where people know what they're doing and are overgeared for, such as level 50 dungeons, usually.