On all EU flights the last 4..5 years the seatbelt sign was only on when turbulences are imminent. Yet, along the common security notice about belts, emergency exits and so on it's always "please keep belts on while seated, even when the sign is off".
For myself I untighten the belt a bit after the sign goes off, but dont put it off completly unless I need to leave the seat. Two occurences of a sudden drop learned me that.