Basically there's two things poison matters for:
Having 3 for horrendous corpse (most of your models that you intend to have die)
Having 5 for blood poisoning (mainly matters for rogue necromancy)
The end of turn poison I consider a bonus rather than something I actively strive for.
Note that to achieve these numbers, you sometimes have to plan ahead (5 poison on turn 1 is down to 3 by turn 3).
Horrendous corpse is probably the most important part of poison in the crew, so I'd start by focusing on that and 5 poison on your rogue necro. Remember that McMourning's actions hand out a lot of poison, so you don't need much.
My next list I'm going to try just doing a single little gasser for poison gen (plus the mindless zombie horrendous corpse poison pulse turn 1).