I don't collect Amiibos, specifically.
But as a game collector and comic collector, I ALWAYS say open it. These things are made to be enjoyed.
I never understood someone sending a comic or a game to be graded, slabbed, and never opened again; for that matter either. I never would.
While I have a backlog, like most people here, I do plan on eventually playing all of my games. I don't buy games I'll never play, unless it's some extra in a lot, and those end up getting traded.
So if I did collect Amiibos, I'm sure I would have the same mentality about it.