I drop Fire Elementals whenever there are a lot of mobs (AoE time) or for every boss fight.
My team is currently Paladin, Warlock and three Shaman.

I have the elementals on their own mapped key.
One press is one, two presses is two and three presses is all three.
And I am glyphed for 5 minute cooldowns.

I've not used Earth Elementals since moving from 5x Shaman to 1x Paladin + 4x Shaman.