if you are heals, i think you just need 100%, but if you are dps, you should get more. your teammates are not going to parry, deflect or whatever you heals, but if u have less than 100, you could miss. if you are dps, your enemies could parry, deflect etc., so u need a lil extra to help ensure contact and that some lightning resistance doesnt get the better of you.
I have 0 accuracy as Sorc heals and have yet to see a heal miss. Can anyone confirm if this is required at all for heals (I have assumed not)