Does FAR Part 121 require at least 45 minutes of fuel remaining upon landing?
No, the regulation only concerns dispatching and taking off. Fuel burn will usually vary a little bit based on unpredicted wind shifts, performance variations and traffic deviations. 


  § 121.639 Fuel supply: All domestic operations. 
  
  No person may dispatch or take off an airplane unless it has enough fuel -
  
  (a) To fly to the airport to which it is dispatched;
  
  (b) Thereafter, to fly to and land at the most distant alternate
  airport (where required) for the airport to which dispatched; and
  
  (c) Thereafter, to fly for 45 minutes at normal cruising fuel
  consumption or, for certificate holders who are authorized to conduct
  day VFR operations in their operations specifications and who are
  operating nontransport category airplanes type certificated after
  December 31, 1964, to fly for 30 minutes at normal cruising fuel
  consumption for day VFR operations.