Dean does not care a bit about you opening his stashes. He might comment on it, but it ends up being something like: "Hey! Those are for emergencies! Well... I guess this qualifies as an emergency but... leave me *something*." Whatever he says is just words. It absolutely will NOT make him your enemy.
I've opened several of his stashes while he's been my companion and it's never adversely affected our relationship (I am always on good terms with him).
The only important thing to keep Dean happy and friendly? Whatever you do, when it comes time to have him take up his position for the Gala, do not threaten him! Threaten him, and you guarantee his enmity.
Thanks for the reply, good to know. Before reading your post I avoided his stashes, just to be safe. I was willing to cooperate on our initial meeting, and activated the Holograms so he could prepare for the Gala. It ended up just fine and he was friendly in the end.
During my fight with Elijah, I think I heard him say something through a speaker or intercom. However, I laid down 7 frag mines, three C4 packages and 8 Gas Bombs as a gift to Elijah, so I couldn't really understand anything of what Dean was saying because of the explosions. Does anyone have any idea what he says, or if he interacts with the battle in any way, like Christine does?