What I see a lot is not flat out lying for the purposes of affirmation, but prevarication--which can be almost as bad. I also think there are some premises to establish with regard to what we think the Tarot is actually capable of doing. I think most people agree, that as the Three Eyed Raven just pronounced in this week's Game of Thrones "The past is written, the ink is dry." I think we'd get into a large debate if we tried the same claim about the future.
Tarot as a Jungian tool to know one's self and promote individuation is therapeutic. Tarot as an oracle is faith-based (faith in deity/deities, in energy, in spirit, in the reader-as-medium, in the cards themselves). I think a lot of people are somewhere in between this spectrum.
We're not licensed therapists, but people may or may not put their faith in us as they would a licensed therapist. The difference is that we don't always have to clean up if we've made a mess. So, I think there's a certain amount of ethical responsibility at play here. Those ethics aren't clear cut or rigid either. I think we have a responsibility to accurately represent what we see in the cards. We also have a responsibility to do it with kindness, as others have said. Kindness does not = prevarication or flat out lying.
Recently, someone on another forum asked me something pernicious like "am I making the right life choices?" That's a whopper of a question. The cards I drew for him were all in agreement, very coherent, very synergistic. It was a resounding and loud "No! Please don't cross the street without holding someone's hand. Or cook. Or do anything without supervision" (this is clearly hyperbole). But when I composed my answer to him, I reframed his question and addressed it as "What aren't I doing and what should I be doing?" so that I could still deliver the message of the cards (stop leaping without looking, you need to think about consequences and Big Picture Thinking, you need to be prepared) but in a way that is digestible. The same information has been conveyed.
I think it's important to read people as well as the cards (divorcing one from the other seems irresponsible, in my view). I'm blunt when needed, careful when needed, loud when needed etc. At the base of this, I think of reading Tarot as helping others, which means framing our messages in a way that has the greatest chance of being heard and processed. However, I don't believe in changing the message. Tone can be 9/10th of communication. I do believe in changing the question beforehand. I"ll flat out say to people "Are you sure this is how you want to ask this? The answer may be as blunt and unhelpful as the way you've asked your question. How about if we ask it in this less pointed way?"
As Marshall Mcluhan said, when talking about communications and media, "The medium is the message." This hits here a lot. The method of presenting information is itself the message. That's what a lot of people will walk away from our tables with.
TL;DR: Don't lie, but don't be a jerk.