To anneal copper you should heat to a DULL cherry red and quench immediately in COLD water.
Copper work hardens and becomes brittle. So for sheet copper work where you bend and shape it, thereby deforming beyond the elastic limit of the material, the annealing process is invaluable to prevent cracks and re-establish elasticity of the metal. The shear, tension and compressive stresses induced in the procedure are relieved. Stress and strain are related by Hooke's Law. The strain is the problem (extension of the material compared to original length). Within linear elasticity, the part will return to its prior form. Beyond that, you anneal.
So injector washers? After torquing the injector, to my thinking, the washers have had one cycle of straining. BUT, the number of strain cycles from heating and cooling is high. Hmmm, think a bit more... the torque tightening is retained between injector changes so the compression load into the washer stays largely constant and most likely within linear elastic limits. So, I suggest that the number of large strain cycles and the amount of strain is low, like once.
So you don't really need to anneal the copper washers, but if you do, it won't hurt them. Torque correctly is the most important.
I wish I could have summarised more succinctly.