Not true. I've have multiple stays where I was told F&B credit doesn't start UNTIL the following day, and I used it as such and the charges were taken off (this is also what I was told upon check in). If the method you described work for you, it might be that the system is intelligent enough to know that a stay gets "x" amount of $15 credits per day and allocates them appropriately. Many MGM desk associates have mentioned that the roll-out and the info surrounded Marriott benefits has been shaky so different employees have given conflicting info to guests, and that if there were any issues with the credit applying, they'd gladly manually adjust it as necessary (they're not worried about giving out a $15 credit). One time, I had an employee checking me in who was adamant that the credit had to be used only at the hotel you were staying at (when we all know, that's not how MGM food/beverage credits work).