This will be really, really simple:
1) When you join a group, ask what the group agrees on - need for companions or greed
2) If your companion needs a piece of gear and your group has agreed to greed, ask if no one else needs it would it be OK to roll need. Just like with alt loot in games where you can trade between toons on your account. Your companion should take priority over vendor trashing it, but not before an actual players character with that rule set.
3) When you form a group, set your rules and let those in group know them. If they don't follow the rule (IE: A tank rolls need on healer gear if you've agreed to greed for companions and they didn't ask) kick them.
IMO one should always roll greed for a companion - any players actual gear trumps a companion who won't be on instance/raid runs in most circumstances. But as long as you make your groups rules clear to begin with, there shouldn't be an issue whatever you want to do.