As a Whm or Ast under diurnal, your going to overheal. Its how the HOT system is pretty much built. I will start casting a Cure/Benific at 80% or if they get down to 60% or I /know/ they're going to be nailed; I instead start with Cure II/Benific II. I always have a Regan/Asptected Benific on my tank; will usually toss that on an injured dps too unless they're ~ 65% or below. If I can, I will keep Medica II/Aspected Helios on tank (party bonus sometimes) are larger pulls or bosses. Does it Overheal? absolutely; but knowing that I dont have to immediately panic if I have to avoid stuff or finish off a dps cast? Worth it. Until I oh shit with Clerics. >.> Then I'm probably spaming Level II spells and praying I caught it fast enough.
That said: Overhealing in XI was nearly impossible (regan lol) WoW, Rift, and a few other mmos which names just escaped me; Overhealing is common. Dont let it freak you out. If you have MP, No One is dead/dying, DOTs are up, your fine.