I'm probably slow to the party, so sorry if this is well known.
Regen- in my opinion, a very awesome spell. But I found that if a tank pulled a group of trash mobs while s/he had regen, the first tic would always bring enmity onto myself, and it was hard for most tanks to get it back. To remedy this, I wait until I cast cure at least 2 times, and then cast regen. Then follow that by a cure 2, and then usually cure 1 is good enough after that.
Hopefully this is helpful to someone.
Share what you have learned, please!