So You Wanna Raid...

[TalAB]

Also, TalAB, overhealing doesn't cause aggro.  Too early healing does.  So far as I know, regrowth doesn't cause more aggro than healing touch.
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Not so much cause aggro or draw it off (though those things can happen as you point out). What it does do is generate secondary threat. How many times have you seen stray mobs go straight for the healers?

I still remember one instance in ZG where a sheep broke early and went straight for Itto, who faded causing it to munch right away on me. The more we heal (and overheal) the more secondary threat we build, this is why Blessing of Sanctuary is an absolute must whenever available - even in preference to wisdom :)