I've booked rooms with F&B credit at MGM properties several times. The one thing that I can say is to read very carefully the terms and conditions of the specific offer you're booking.

As an example, on one occasion, the food and beverage credit had to be used prior to midnight or would vanish. On a different one-night stay, the F&B credit became available only at midnight, so I could only use it for breakfast the next day.

Those are just two examples of experiences I've had. Don't ask here on FlyerTalk; read the terms of conditions of you specific offer. In my experience, credits are applied automatically every day by the computer (which you can confirm on the in-room TV), but I cannot say how flexible the front desk is in adjusting the credits at checkout.