Why are people consistently making threads about this? lol.
Regardless I keep my fairy basically between the range DDs/myself/melees if they are on the back of the mob or whatever. Basically she heals the parties little booboos and I heal the tanks big booboos and we go along with our day. Sometimes I get her positioned well enough to help with all the booboos depending what content I am doing.
In AK Fairy does all the healing though because going DD and killing bosses like the wall before the bees spawn or the purple balls on the final boss makes it faster and easier as long as she is placed by the party during fights its okay.