if you look at a combat log of almost any fight, our rejuves are almost always overheal. The whole point is to have the hot ticking for when the person does take damage. Because when they do, a single crit rejuv will take a large chunk out of that health defecit.
The last world of logs my guild posted shows 70% or greater overheal on every single one of my hots though the entire night, yet I'm still #1 in total effective healing done for every single fight. So thinking in terms of HoT overhealing isn't very applicable to resto druids.