Where do you stand?
First criteria is always: so that everyone getting heals is in range – or will be in range when the spell goes off. This means I may find myself inside melee range of a battle so that another battle’s participant can be healed. So be it.
Second criteria should be: so that threat generation is minimized. Remember that if I’m inside 15 yards of a mob, I need only need to exceed 110% of the highest threat to become its priority, while if I’m outside that range I am allowed 130%. Remember that a mob that’s crowd controlled is a lot harder to spot and stop than one that’s in combat. Adjunct point: a sheeped or trapped mob has the player that tagged it identified as the top of its table, and their threat is artificially adjusted to be equal to the highest actual threat. And 99% of the time that is the threat from MY healing. If I pop a 4K greater heal I’d better have already done 40K of healing across the party already, because I’m the ONLY thing that’s been generating aggro on that mob.
Third criteria should be: so that if I’m attacked it can be seen and fixed most effectively. Often this means I want to be separate from everyone else so there is no doubt if it’s me or anyone else. And if my designated rescue force is a ranged class, I need to be far enough away that s/he can do maximum damage.