For this answer, we turned, naturally enough, to the Queen of Comps herself, Jean Scott, author of our Frugal Gambler series of books. Someone might know more than she does about players club cards, but we haven't met them lately. Here's her response.
It really doesn't matter, in most cases, whose card is in the machine when a W-2G jackpot ($1,200 and up) is hit. The casinos issue the W-2G to the person sitting at the machine, whose fingers hit the buttons that brought about the payout that requires the tax paperwork. The jackpot will never be voided and you usually won't get into any trouble.
There are many reasons why the jackpot hitter and the players card might not match. This happens to Brad and me every once in a while. Brad hits a jackpot and I decide to take a bathroom break, so he slides over to my machine, which has my card inserted, and starts playing it while he's waiting for the pay on his machine and me to return. If he's is lucky enough to hit a big jackpot there, that's not a problem. He gets the W-2G even though my card is in the machine.
Another example: A player forgets his card when he leaves a machine, then someone else sits downs and starts playing, not noticing that a card is inserted. Whether this is a person who doesn't have a card or one who just forgot to put in his own card, if he hits a big jackpot, he's the one to get the W-2G.
When a W-2G jackpot is hit, that event shows up on a computer somewhere in a "back room" of the casino. That not only shows the location of the machine and the amount to be paid, but information about the person whose players card is in the machine as well. In very rare cases, employees have made out the W-2G in the name of the person whose name is on that players card, then had to void that W-2G and make out another after finding out that this wasn't the player who hit the jackpot. However, because gaming regulations are strict about giving the W-2G to the actual person who hits the jackpot, overwhelmingly, casinos personally ask for ID before they make out a W-2G.
Most casinos do have in their players club rules that everyone must play on his own card, though most of the time they don't enforce it. Couples playing on each other's cards is quite common. In fact, a few casinos link accounts of couples who share the same address, although the trend nowadays is toward individual and unlinked accounts.
Mainly, you might get into trouble with the casino by playing on someone else's card, even a spouse's, during a special casino promotion. A common example here is a slot marathon where multiple people playing on one account would give a definite and unfair advantage. This is a good reason why you need to carefully read and follow the rules for any promotion. I've known many players over the years who have been 86'd from casinos and most of them have been guilty of participating in promotions with players club cards that were not their own.
You do have to be careful in this area. You can go to my blog and read an entry related to this subject, one that tells about a scary experience Bob Dancer recently had with this subject.
