When I have a spectator select a card, I ask them to show it to everyone around them. First of all, that helps if they "forget" which card was theirs. (I try to make my tricks short enough to where that doesn't happen, but it still could happen). AND it helps keep them honest.
[Paraphrase of an actual conversation I had one time ...]
[Magician] "And this is your card!"
[Spectator] "Uh, no, it's not."
[Another audience member] "Yes, it was, Johnny. Don't give him a hard time." Turns to me, "Sorry -- he just likes to be difficult."
He thought he was being "cool", he was going to act like he caught you, but he got caught instead.
Besides, if the rest of the audience doesn't see the card, they don't feel like a part of the action.
Just my 2-cents worth.