Well, I am going to take this a bit differently. I am placing some of this upon the querent. I do truly believe that even if only held in our subconscious minds, there is some level of knowing the answer/outcome that is immediate. So if in a party situation and a querent ask a question whether seemingly heavy or not, I believe that there is some knowledge of the believed true outcome or response of the cards/reading. If they are strong enough to ask a question that leads the cards to the drawn in a more serious outcome, then give it. I go with giving only truth, but yes that it is with tact and gentleness, without sugarcoating it or handing out false statements. The time frame would factor in. Can this reading be done gently and cat and dry both in order to move on for other readings? If not, would go with LucyD's way of giving them a card to call for an in-depth reading for a discounted price. I think you would have to say something like "This is far too involved to give this it's due in this venue. Call me when you have the chance". Then ask if they have something else that want to know that would be rather quick, since you are cutting into other's time.
I just know I would want whatever the Reader saw from the cards. Besides, two other factors to consider...1) What if this reading is the first reading the querent ever had? It can't be all cotton candy funtime if it is dire and you try being light and happy totally with the reading. Do you want them to go away with the believe this is all a gimmick, "What does she know!", and there isn't anything to it? ...2) Take it that all readings you do no matter the event is a reflection of how good of a reader you are. If you are trying to build up your client base, what is this going to do? Help or hinder?