While they were similar in their power of showing the viewer future events, I think the Palantíri were more dependable. Galadriel said herself:
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Remember that the Mirror shows many things, and not all have yet come to pass. Some never come to be, unless those that behold the visions turn aside from their path to prevent them. The Mirror is dangerous as a guide of deeds.
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FOTR The Mirror of Galadriel
Compare that to Gandalf's statement about the Palantíri:
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The Stones of Seeing do not lie, and not even the lord of Barad-dűr can make them do so.
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ROTK The Last Debate
Galadriel may have dared to try and view Dol Guldur, but could she have been certain if what she saw was accurate?
As to why the Ithil and Orthanc stones were not used to divine the Keepers of the Three, I once considered that in order to look on a
place with a Stone, you had to have both an idea of its specific location and its appearance, otherwise you were merely 'shooting in the dark'. Also perhaps if Sauron or Saruman did attempt to view Rivendell or Lórien, the Rings themselves may have been a protection.
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I say to you, Frodo, that even as I speak to you, I perceive the dark lord and know his mind, or all of his mind that concerns the Elves. And he gropes ever to see me and my thought. But still the door is closed!
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FOTR The Mirror of Galadriel
Actually, it's odd to me that Saruman wouldn't have known already where the Three were bestowed. After all, he had been the head of the Council, and probably had the most knowledge of Ring-lore of any who remained in ME, save Sauron. Moreover, he was considered one of the greatest allies of the West for most of the Third Age.