It doesn't matter what boss fight you are doing.
If somebody starts a pull timer, Respect it.
Some people like to optimize rotations, waiting 15 seconds for the tank to pull isn't going to kill you. Pulling before the tank certainly will.
Dungeons? Go ahead, bring the trash to me, ill tank it.
But if your going to prepull in a raid/trial when there is an active pull timer, then your only griefing yourself.