A measure of common sense
My experience when flying as SLF is that short, clear instructions which seem to have a purpose achieve better compliance than broadly scoped "rules". For example, explaining to SLF that they can't take a large bag on board because it won't fit is better than stating that the bag size "exceeds company policy".
It is the same with SBs. If pax know why the sign is on they are more likely to follow. They know that on takeoff and landing there may well be "bumps" so comply. persuasion rather tha legal sanction will deliver a greater likelihood of the belt staying on when the sign is off. keeping the signs on for longer doesn't achieve anything for safety or goodwill. I have seen crews explain the benefits of keeping belts fastened and noted good compliance. I have also seen cres demanding compliance and being sworn at.
My crews keep the sign on for as short a period as possible, commensurate with safety requirements.
If anyone on this thread thinks that an announcement to keep belts on throughout the flight is going to prevent legal action in the event of CAT and injury, they are living in some land way, way beyond the clouds. If a pax is injured, s/he will sue. That's it.