Last note: I see the saying the truth but bending the meaning to be polite as signaling to someone that you don't quite mean what you're saying subtly enough that if (and only if) they care about your true opinion enough to pay attention to what you say and ask a followup question you'll tell them the full story. If they were just looking for a generic nicity, they either won't notice your slightly careful wording, or should not request information they do not want. This is useful for people who may have reason to want your true opinion, and as a way of avoiding getting into the habit of telling white lies.

I think this is a great point. By verbally giving positive feedback, and nonverbally giving lukewarm feedback, you are not necessarily lying, because your communication is not just your words. If someone wants you to give a comprehensive critique, they can ask for it explicitly. This way, the people who want encouragement can get it, and the people who want critique can get it.

To me, the most intelligent default is that I consider a request for feedback to be a request for encouragement, but people can always override this default by explicitly asking me for a critique.