I would be careful about applying this principle to Plato because he himself advocates lying to students, insofar as you can take him at face value -- and if you can't, well, here we are. Plato seems to have been a smart guy with an agenda and no qualms about manipulating his readers. That it's manipulative doesn't mean it's false or worthless, but I won't read it in the usual Gricean way.
Does Plato actually advocate this? If you are referring to an argument Socrates makes in the Republic regarding politically motivated lying, it seems to me not unreasonable at an initial glance to say that Socrates represents Plato's position.
But it also seems to me that there are other ways to take this. First, the Socrates in the Republic is a character in an exchange — and so it seems not unreasonable to think that perhaps that character is not a simple mouthpiece of the author. For example, nobody would say Hamlet is Shakespeare's mouthpiece. Second, there are dialogues where Socrates gets completely trounced by his opponent — in particular, the Parmenides. Interpretations of what is going on in the Parmenides are diverse; but at the very least it seems that Socrates is not Plato's mouthpiece. Third, if (contrary to the previous two points) it turns out that Socrates is Plato's mouthpiece, but that mouthpiece is telling us that Plato will manipulate the reader as he sees fit, then it seems at least possible that the whole idea that Socrates is Plato's mouthpiece is itself not a straightforward claim. Thus, the grounds for your claim that one should be careful about applying this principle to Plato — assuming I am right that you base it on evidence in the dialogues — is not entirely solid.
As an additional point, it seems to me that even if the author wants to manipulate the reader, it still stands us in good stead to have a principle of charity. It is a starting point, not an ending point.
Yes, it's a passage in the Republic that I'm particularly thinking of. I have only the vaguest memory now, something like that only a few of the oldest and wisest should be told the real reasons for rules, and the rest should get various levels of cover stories?
Agreed that there's a deliberate indirection in dialogs; that's why I included the caveat. But to take another example, the Turtle is not always Hoftstadter's mouthpiece, yet Hofstadter doesn't seem especially manipulative to me. Plato (at least in translation) does. The passage I brought up just crystallized that reaction. I started out reading Plato from the usual charitable standpoint, and ended feeling that's a mistake: whatever's he's up to, it's not primarily to help the reader become a better independent thinker, or to accurately report events.
I've read less Aristotle, but he gives a different impression: someone with faults like overconfidence, but who's honestly pitching in to the project of improving collective knowledge and thinking.
