I have a go to magic effect that I have performed thousands of times, I "slip the aces into the deck" and ask the spectator if they believe the cards are lost. The answer is and always has been yes, never once have I had anyone ask to shuffle. So, I have some issue with the assumption that ANY spectator would want to shuffle, because I have had a different experience.
Josh:
Why do you ask the spectator that question? If they believe the cards are lost, having them say it doesn't make them believe it any more. However, it does, by negative implication, suggest a method -- "what if the cards aren't lost in the deck?" If they don't believe the cards are lost because you've done some of that hocus pocus, prestigiditation, "hands are quicker than your eyes" thing they will either lie (being polite while thinking the cards aren't lost) or challenge you.
If I was doing something like that, I would ask them "do you have any idea where the cards are in the deck?" The answer is going to be "no." My response would be "neither do I... that is what makes this interesting."
My script for one effect is as follows:
Me: Do you play cards?
Spectator: Occasionally.
Me: Do you always win?
Spectator: Not always.
Me: Do you cheat?
Spectator: No.
Me: That explains why you don't always win [this gets a laugh from the spectator and the audience].
Me: I want you to shuffle this deck till your heart's content.
Spectator [shuffles the deck]
Me: Are you done?
Spectator: Yes.
Me: [in a sincere voice] Is your heart content?
Spectator: [pauses, smiles and then says] Yes..
Me: Perfect. Perfect. I would like you to select one card, it doesn't matter which one you take.
Spectator: [selects one card]
Me: Can you tell everyone what you picked and show the audience your card.
Spectator: Ten of Diamond [as they show the card]
Me: Please put it back in the deck.
Spectator [puts it back in the deck]
Me: Can you hold the deck, I don't want to even touch the cards. Actually, put the deck pack in the box and hold on to it
The audience remembers that the spectator shuffled the deck, picked a card, put it back in and then put the deck in the box. They are thinking the trick is about to begin... but everything is already done. I don't ask "do you want to change your card" or "is this fair" and I don't make any statements "you could have picked any card you wanted but you freely selected one card" or "the deck is really shuffled so there is no way I could anyway influence your selection?" It just isn't necessary.