Is it always advisable for passengers to wear seat belts when a 747 is on final approach?
Yes, it is always advisable for passengers to wear the provided safety belts in all phases of flight. This is especially true for aircraft on takeoff, landing, in low level flight, or in any other flight regimes that are especially prone to turbulence or other risk exposure.

A large, heavy aircraft—such as a B747—will be less affected by wake or mechanical turbulence than would a smaller aircraft—such as a CRJ200 or a C172. However, even large, heavy aircraft can be subjected to severe turbulence that could result in injury or death if passengers are not restrained by appropriate safety belts. 

Additionally, for flights conducted under 14 CFR 121—which I am confident is a majority of the US operated passenger flights involving the B747 (though not, obviously, Air Force One pictured in the question above)—passengers are generally required to:


  occupy an approved seat or berth with a separate safety belt properly secured about him or her during movement on the surface, takeoff, and landing
  Excerpted from 14 CFR 121.311 (b)