Right now, GCure pots cure DP every time.Higher poisoning allows you to apply deadlier poisons. GM poisoning gets a 40% boost over 20% boost to proc your poison per hit iirc.
Deadly poison can be great for PvM, and sometimes GCure potions or spells have a hard time healing deadly poison.
Most believe that 50-70 poisoning for regular poison/greater poison works for them. To each their own