As a healer, where should I stand in dungeons?

[Kusumura]

Depends on what the monsters do. Most healers are just flat out scared to stand close, but if the monster doesnt do anything that will hit me or get others hit if it has target aoe attacks, then I will stand very close to the monster.

You just gotta know your monsters. There are even those fights where it is detrimental to stand far.

Actually, very frequently in most games, you're a lot safer being close enough to sniff the arm pits of the hour's Big Bad - or, their farts, anyway, since being behind them is much better! Why? Anything that may aggro onto you will just be swept up as soon as the tank uses an AoE. You don't have to worry about avoiding too many offending AoE's - although, those that may be point blank might require you to actually take an action. But it isn't that hard.

Instructional video for those that are not aware of the method to not dying: Taught by master Piccolo himself.

[Suzushiiro]

You stand where everyone is in range of your heals (and will continue to be in range of your heals, when talking about encounters requiring a lot of movement) and your survivability is highest. This means not in the fire/cleave but also means not so far from the tank that it takes forever for him to grab a mob you may pull off him by healing aggro, and not so close to him that he can't easily tell that you've pulled aggro on something because despite being on you it's still mixed in the pack with the rest of the things beating on him.

[16bitBounty]

I stand back unless the mobs are harmless. If I pull something I run for the tank so they can take it back.

[tibberton]

One thing to consider for positioning, too, is line of sight. Pillars have dropped line of sight. The bonfire in Halitali dropped line of sight. It's a key thing to consider in regards to positioning.

[Navine]

In the back where you belong hah! For me it really depends on the boss. Some bosses have frontal aoe so I stand to the side but given enough distance to heal and reach everyone. You just gotta find what works for you.