It really doesn't matter how effective or justified it is. Simply put, your a DPS and you are not supposed to DPS until tank is set and starts rotations. If your casting as soon as the tank tags a mob for a pull then your at fault regardless of what the tank does.

The tank (right or wrong) has the role of placing the mobs where they will be killed. Any action prior to placement or setup by tank puts you as a DPS in the wrong. It sucks sometimes with unknown tanks but that's been a DPS rule in mmo for over/about 15 years.