Read an article way back that Alaska Airlines were certified to use side slips into specific destinations. At the airlines I have been never been certified nor mentioned in the manuals. Tried it in the sim though. Airflow changes and other events may have an interesting affect. Should always be able to perform one though as you never know when you might need it.
Nomenclature:
Side slip - longitudinal axis is parallel with centerline in cross-controlled situation (x wind landing)
Forward slip - longitudinal axis is not parallel with centerline in cross-controlled situation; more rudder than aileron (increase rate of descent without increasing airspeed)