As Mesarthim said, Room size and party location. If the entire party has to stack in a certain area for a mechanic for example.
But in general, mid is a good spot.

However... and especially in 24m raids, arena are sometimes really wide, and your caster/ranged dps will probably for a lot of them be so far away that your fairy's abilities (dawn, fey blessing...) won't reach them.
But you can't place the fairy accordingly to their random positioning (since they ignore healers range). But just keep that in mind that you can't totally ajust her position for everyone.
Mid is often a good place. Just never let her follow you automatically.
I don't know what she's doing right now, but before, a fairy following you each time you move was a fairy who couldn't cast her embraces peacefully.