Dear Summoners...

[Nerael]

SMN, WAR, WHM here.

My perspective from each role:
SMN: Always keep Pet on Obey, Macro certain pet skills into normal skills (I have obey weaved into ruin II so I know Garuda attacks same target I am). Bane with Aerial Slash for that little boost of AOE deepz.
When tanks are not holding aggro or a tank is down, Titan to the rescue. I have had titan tank the Cerby adds as well as in Belly he tanked (We were one tank down).
As long as you don't bother the tank, MANAGE your pet there is no issue. Soon as Titan was finished? Welcome back Garuda, I missed you buddy!
If you have aggro only use Garuda's push skill as last resort. First try to get the mob to the tank so he can grab aggro. If the tank still does not grab it off you? That's when you can use it. Do not use it if the tank is trying to grab aggro or you're just making it more difficult for him and yourself.

WAR: If you lose aggro to Titan I will point and laugh at your mercilessly. Besides, you are gimping your DPS with titan out while you have a proper tank, why use it in the first place? You should use it when there is a lack of tank or if a tank is not doing his job. Titan should not be used otherwise (Last Resort). Also don't have Ifrit use stun skill or I might not be able to Stun/interrupt when I really need to. Pushing away mobs is also stupid. It takes me longer to get aggro.

WHM: Keep adds away from me. I try not to grab aggro, allas Medica II is sometimes required and sadly enough it creates a lot of enmity. Shroud is not always available. Preferably the tank grabs aggro on the mobs. If the tank cannot, I definitely appreciate the SMN doing all in his power to keep me safe (and thus keep the group alive)