You do not have to choose which restaurant(s), or when, you will utilize those FAS credits pre-cruise (can be done onboard).
When you select the Free at Sea promotion, your booking has service charges applied to your invoice. If you decide that you don't want the Specialty Dining, &/OR, the Beverage Package to be included, you can cancel either of those items pre-cruise.
HOWEVER, if you decide you don't want something, it is best to cancel those items 3+ days prior to your cruise date.