DTH
You will find a varying degree of paranoid use of seatbelt signs. Some will turn them on with a fart while others seem to wait until the acft is upside down before the consider it.
Also at certain times it may be more prudent than others to be over cautious such as during hot food and beverage service while other times during "sleep" it may be left off to prevent that "unnecessary" PA.
A bloke in America has just been awarded over 2 million dollars for being blind drunk and falling into the path of a subway train injuring himself in the process. This story itself shows the world we live in and maybe you can appreciate that having a more cautious approach sometimes may help in the ass covering later.