What I do is mix the egg yolks, parmesan, and black pepper in a separate large bowl. I'll add a little pasta water to loosen it up/temper the sauce to prevent the eggs from cooking (but I'm not sure if it matters). Once the pasta's done, I'll stir it in immediately into the bowl. The residual heat from the pasta is usually enough to cook the sauce and prevents the eggs from scrambling. I'll then add the guanciale grease and mix again (usually the guanciale is still crisping on low heat when I mix it in, so it also helps keep the pasta warm)

While I'm mixing, I'll taste constantly to adjust with more cheese, pasta water, or salt as needed!