Personally for myself I don't think there was any need for the name "Sith" to be mentioned in the original trilogy. Palpatine by this point had already cemented his rule and become the tyrant he was with Vader as his enforcer and Tarkin his military commander. As I mentioned before people could think what they wanted about Palpatine at this point of his reign..there was never any need to mention that he or Vader were Darth Lords of the Sith. I don't think the citizens of the galaxy ever really realized what he and Vader were. WE knew what they were, but they did a good job at concealing themselves from the galactic populace.