Not a "hard" rule but in general for dungeons most dps do better with Thaumaturge or Biting Aura for dungeons and Backstabber for trials. Positioning in dungeons is harder and fights are quicker so unless with a dedicated group or tank you know very well Backstabber is to hit or miss on bosses and almost always useless for trash. With the new crit damage cap and various builds even in trials going to have to weigh it's usefulness to your specific build and group composition.
Funny thing, after made some test it seems that in most 4-man content i am doing around 135% crit damage, so it seems that bckstabber is ok, but the CP with 10% crit dmg must go out and be replaced with dmg CP. And i have seen that in Necro build at Alcast website they did the same
And about Canceroid - i am also wonder. Try to test it with Dragonguard Elite, if i will finally manage to go solo to Marcy next time
I would just drop BS for LoM anyway. You are not going to be able to use BS on the final boss as is, and 99% of the time nobody attacks the back of the dragon in the strangler encounter.