[Rumil]

I think Sauron was only called the Necromancer during his time at Dol Guldur when his identity was uncertain.

I'd guess he did practice a bit of Necromancy every now and then. The Nazgul have been mentioned, and remember that even the Witch King managed to recruit a bunch of Wights to infest the Barrow Downs. Exactly how is not made clear, houseless elvish fea, victims of Morgul blade stabbings, plain old-fashioned zombification, who knows? One thing to remember is that the books were supposed to have been written by the hobbits and even if the Wise knew how such things were done I think they would not have wanted to publicise the methods.

One passage from LoTR has also struck me in this respect. I can't remember offhand who says it, but it goes something like - 'Marvelous we deemed it that he defeated Sauron's forces with his own weapons'
This refers to Aragorn's use of The Dead to retake the Pelargir from the Corsairs. Therefore, some at least associated the undead, ghosts, whatever, with Sauron.