I have noticed since the last patch it seems a bit easier to pull aggro off the tank. But it does really depend on the tank, and on the group.
Some groups I can basically chain-cast everything non-stop and never get aggro, other groups I cast BSS or Null Absolution once and i'm covered in MOB's in the first seconds.
I usually start slow and work my way up until I draw it once or twice, just to find the line. I'll warn the healers and tank in advance though that I'm likely to pull aggro a few times before I find the good balance.
btw Null Absolution seems to be the absolute worst hate producing spell we have. I have the adept I and it's doing close to 300 direct damage, and then another 200 damage over the next 8-9 seconds. If I use this in the first few seconds of a fight it get's ugly quick heh.