I recently booked specialy dining in the Crown Grill for six people.
I thought my on board credit would be used for this.
However, Princess immediately charged my credit card. . I only discovered it as I monitor my credit card charges often and it showed up on my statement right away.
Waiting to book onboard did not work on my last cruise, as this specialty restaurant was completely booked since it was a short cruise.
So this time, I made the reservation weeks in advance of the cruise, but would have preferred to be able to use the onboard credit to cover this speciality restaurant.