For dexxers, the higher the poisoning skill the higher the percentage of hitting for lethal where in the past for mages the conventional wisdom was as long as the combined total of the two skills totaled 200 points then you were good to go. As far as 80 or GM, it depends on if you can spare the points. I have some chars that 85 is about all I could put on and other templates they have GM, in my opinion, I don't see that much of a difference between the two on a dexxer...then again I'm usually to busy keeping track of other things to keep track of the message indicating what level they were poisoned.
As far as necro, this is the notes from publish 71: "Poison Strike (Necromancy) : Necromancers with greater than 60 poisoning skill will have a 3% to 9% chance (Scales with poison skill) to poison their target."