To add to what
@Antonio Diavolo said, the line confuses spectators and distracts from the effect. The spectator isn't sure if they picked the wrong card, if they should pick a different card or if they should stay with the card they picked. Every time I see this said, the spectator looks uncomfortable. Further, it distracts from the effect by putting more emphasis on the selection than what is needed. If the method used for a selection is fair, there is no need to try to reinforce it. The more you talk about the fairness of the selection, the more the audience thinks about whether the selection is fair.
Finally, I'm a big fan of showing rather than saying. Show through actions that the selection of anything is fair. When you have to say something to establish a selection is fair, the natural tendency of the audience is to question what you say.
Also, that line is usually followed by "OK, then you'll keep the mind you have..." Sigh.