Any unattacked/untaunted mob is going to go after the healer. It does not matter if I have 100% present threat or .00000001% threat, if they are untaunted and unattacked by everyone else, then .00000001 is still more than 0.000000000, so reducing it means nothing. More important is for dps/tanks to know kill order and their role. Now I can use disapearing act to drop combat, but the moment I unstealth and pop a heal they are back on me.
Gold Fever a killer of Healers.
As to threat droup by dps, yes that should be part of the roation in group play, especially during/after opening.
As someone who primarily tanks, this above everything else except do not pull before the tank. Since you are doing DPS (and should stick with that until you are comfortable with the game), 97% of the time your first targets should be the weakest enemies while the tank holds aggro on the strongest and as many of weaker as possible.
If you focus on the weak enemies first and quickly switch to mobs attacking a healer, the tank and heal will love having you and you will (almost) never get kicked. Always will be a few jerks out there.