He's asking about 135.267(f): The certificate holder must provide each flight crewmember at least 13 rest periods of at least 24 consecutive hours each in each calendar quarter.
The answer is yes, you must take them. If you don't take them, that is, the CH allows you to come on in and work anyway, then the CH has not actually provided the rest periods.
Additionally, the 24 hour rest period must be free from all obligation to the CH. The pilot cannot be on-call or on standby. The pilot cannot be working on paperwork required by the CH. The pilot cannot be undergoing training required by the CH. Etc. "Rest” is defined by the FAA as a “freedom from any obligation” to the Certificate Holder.