I will preach the obvious and state that you must always use parts that the AMM and IPC specify. In the case of flight and engine control always use new stiff nuts every time that they are disturbed .THEY ARE ONE TIME USE ONLY.Another thing to consider is that of the source of the stiffnuts. In the case of engine and flight controls I would only feel happy using parts supplied by the OEM. There are many standard part suppliers out there and bogus parts are not unknown and difficult to spot even to the trained eye. A known history is everything.