I think the whole point of The Emperor/Sidious is that he deliberately hides his true self and his powers, and manipulates things behind the scenes and gets others namely Vader to do his dirty work for him, Outside of the Jedi council I'd be surprised if anyone else knew of the existence of the Sith or what a Sith lord was, he became Emperor by discrediting the Jedi and ending the Clone Wars, by the time he ascended to power he would have been seen as the Hero who ended the war and bought Peace to the Galaxy which opened the floodgates to corruption, after all it did take take 20 years for a small band of rebels to make any sort of effective stance towards the Empire.

I always assumed the Emperor was a reclusive leader that only a precious few high level Imperial dignitaries would ever come into contact with, bueraucracy would do the rest. I'm sure many would suspect he was pure evil, but I think he would have kept his dark powers well hidden operating only to subtley manipulate events on a large scale, Vader was his show-pony and his display of powers were enough to instill fear and obedience throughout the Galaxy.

unfortunately the EU only tends to muddy this issue by making his Dark Powers obvious to all.