I know you can still loot from standard boxes after someone else has opened them - on my Rogue I recently sold a junkbox with a BoP jewelcrafting recipe in it, and I routinely leave mana pots in locked chests for the mana-using classes.

I am pretty sure you can loot the Ramparts end box even if you are not there for the kill but this may have been changed recently. I'd like to know for sure though.