Why not if you've had problems with mould before. Better to fix the underlying reasons for the mould though. Also clean the ceiling with bleach if there's any existing mould (not sure how new the en-suite is).
It's not always a requirement for a specialist paint like that. Sometimes it can complicate things unnecessarily. If you have good ventilation there's a good chance the ceiling will never get mouldy. I've painted quite a few en-suite after many years of use that don't need it.
Why not if you've had problems with mould before. Better to fix the underlying reasons for the mould though. Also clean the ceiling with bleach if there's any existing mould (not sure how new the en-suite is).