Do I have to declare an emergency if I cut into fuel reserves? In another question, I wrote

You only need to plan for regulatory reserves. You don't actually have to land with them

and @Jan replied:

you have to declare emergency if you do cut into them though and that will quite probably get you an investigation

I have never heard that you must declare an emergency if you cut into your reserves (regardless of the stage of flight), and I'm wondering if there is any regulation to back it up.
The only reg that I could find that comes close is NTSB 830, which lists a set of criteria for notifying the NTSB within 10 days:

(a) An aircraft accident or any of the following listed serious incidents occur:
(1) Flight control system malfunction or failure;
(2) Inability of any required flight crew member to perform normal flight duties as a result of injury or illness;
(3) Failure of any internal turbine engine component that results in the escape of debris other than out the exhaust path;
(4) In-flight fire;
(5) Aircraft collision in flight;
(6) Damage to property, other than the aircraft, estimated to exceed $25,000 for repair (including materials and labor) or fair market value in the event of total loss, whichever is less.
(7) For large multiengine aircraft (more than 12,500 pounds maximum certificated takeoff weight): (a bunch of other things)

I'm interested in the answer for Parts 91, 135, and 121, under FAA regulations.
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)