'Recommended' only, and part of the Passenger Briefing requirements in EUOPS, but in no way at the 'discretion' of the PIC.
After take-off
1. Passengers are reminded of the following if applicable:
(i) smoking regulations; and
(ii) use of safety belts and/or safety harnesses including the safety benefits of having safety belts fastened when seated irrespective of seat belt sign illumination.