As a Tank, please do not think you have an excuse to "Stand in the Bad." Yes, tanks are technically more well-equipped to handle avoidable damage then DPS, but that is not an excuse to take avoidable damage. Each cooldown spent on avoidable damage is one not spent on a tank-buster, large pull, or even just giving your healer some breathing room. You are literally wasting both your own cooldowns as well as your healer's limited mana pool/cooldowns. Plus, it's just rude toward your healer. Do not make your healer's life harder than it needs to be.

While positioning is of course important, minimizing damage taken is even more. Keep in mind, when you dodge an AOE, the melee DPS performing the positional is probably going to have to dodge that AOE as well. They'll have to re-position at the end of the AOE either way, so you won't really be helping them much. It's better to dodge the AOE and give your healer some time to DPS. While mistakes certainly happen, you shouldn't purposely stand in avoidable AOEs.