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.