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.
