When asked to roll at the looting window for Need or Greed please bear these in mind.
The item is one you need NOW, that is you can equip it there and then and gain benefit. Everything else is GREED.
If an item can not be used by you but by an alt then it is Greed,
If an item can not be used by you but can be sold on AH it is Greed.
If an item can be disenchanted by you then again it is Greed.
If it is a gemstone or ore then it is Greed regardless of whether you are a miner or blacksmith.
These are items that are bound on pickup, ie. become souldbound and useless to anyone else as soon as you win the roll. You should enter NEED or PASS for these. If everyone passed then the party members can /roll for it and the winner can take it. In this case you may ask an Enchanter in the party to take it and disenchant it and give you the materials.