As I understand it, the rule is:
Need: If you need something for the character your are playing with
Greed: If you want something for offspec or if one of your companions could use it
Pass: If you have absolutely no use for it because neither you nor any of your companions could use it (like Sorcs and Aim gear or Consulars and Strength gear) and you are rich enough to even care about vendoring it.
I might occasionally greed on things that are BoE so that I might sell them on the GTN, especially on empty orange shells, but I will never need on anything that's not an actual upgrade for the gear my character has.
the only time i pass it for guild run or if the gear is soo low i rather have the space then the credits it worth, i greed whatever i don't need for myself. getting gear for off spec and companions is easier questing and planet storylines and not part of my main focus.