Perhaps at the time when Aragorn said 'Neither does he use his right name, nor permit it to be spelt or spoken...' at the time Sauron was still building his power and armies trying to be as secret as possible. Then by the time they speak with the Mouth of Sauron (third book) its already obvious that it's him so why pretend it's not.
Here's something out of the Encyclopedia of Arda that I base my statement on.
"One possible reason for
Aragorn's error is that his information is out of date. For most of the
Third Age,
Sauron had been building his strength, in secret, at
Dol Guldur. Given his need to remain hidden, it's natural that he would ban his servants from using his real name. Any detailed information that
Aragorn had about him and his ways would date back to
Gandalf's spying expeditions in this period."