Using salted butter in graham cracker crusts elevates the level of flavor in the entire pie so significantly, that I just can't not do it. It's the same reason I use salted butter in my cookie doughs (see my Salted Butter Pumpkin Snickerdoodles for proof). And it's the same reason I like salted butter in butter creams. The salt deepens and strengthens the overall flavor and punches back against the sharp sugar-smack of the sweet things we're using. It's all about balance.