Do I have to declare an emergency if I cut into fuel reserves?
There seems to be no regulations that require decleration of emergency in case of using reserve fuel. US DoT InFO 8004 specifically discusses this:


  Emergency Fuel. Although not defined in the AIM or Federal aviation regulations, the  industry-wide connotation typically associated with the 
  term “Fuel Emergency” is:
  
  The point at which, in the judgment of the pilot-in-command, it is necessary to proceed directly to the airport of in tended landing due to low fuel. Declaration of a fuel emergency is an explicit statement that priority handling by ATC is both required and expected. 


Noting that pilot declaring minimum fuel is not an emergency,


  The act of using a portion of the reserve fuel assigned to a flight is not,  in its self a cause to declare a minimum fuel state with the controlling agency. Regulations require reserve fuel to enable aircraft to maneuver, due to unforeseen circumstances. Many aircraft safely arrive at their destination having used a portion of the fuel designated as reserve. There is no regulatory definition as to when, specifically, a pilot must  declare “minimum fuel” or a fuel emergency. Air carriers typically develop such guidance for their pilots and include it in their General Operations 
  Manuals; such guidance generally  falls along the following lines:
  
  •Declare “minimum fuel” when, in your best judgment, any additional delay will cause you to burn into your reserve fuel.
  
  •Declare a fuel emergency at the point at which, in your judgment, it is necessary for you to proceed directly to the airport at which you intend to land. Declaration of a fuel emergency is an explicit statement that priority handling by ATC is necessary and expected. 


(Emphasis mine)