I've been thinking about this one for a while... the truth is, personally, I don't usually use "tu" to address a superior at work, but I have used it to address people working under my management. More than anything else, to present myself accesible to them. Now, I did have a few Spanish (from Spain) managers who would address all of us with "tu," in most cases I replied in the same way, but there were a few cases where I realized that "tu" was perfectly fine and we wouldn't loose our respect, professionalism and understanding of who was in charge.