I've been seeing a lot of this lately. If I get a good tank, I can heal like mad and never steal agro. Today, I've been getting nothing but bad tanks. They either don't flash or only flash once. In short time, the adds are attacking me. The DPS try to help get them off (probably a bad idea), end up getting attacked, I'm healing them now, generating more threat.

If I have a tank that flashes a couple times at the beginning and actually keeps an eye on agro, there's no problem with threat from my heals.