If I have negligible plaque in the carotid and peripheral arteries, do I need to get a carotid ultrasound?

??? I am confused. How do you already know you have neglible carotid plaque without an ultrasound? Was it by ct angio? If there is reason for concern, bruit, stroke sx, it may still be of benefit.
Yes. If you know you already have plaque formation to your carotid vessels or peripheral arteries at your age of 42 yo - it would be wise to focus on the risk factors that led to this development (high cholesterol, high blood sugar, high blood pressure, smoking) where applicable. Certain medications such as statins can stabilize the plaque, prevent further build up, and even reduce the plaque.
Depends. Even negligible plaque will have the occasion to worsen with time. If you have mild carotid disease you should have a carotid ultrasound at least once year. This may increase to twice a year if there is more significant amount of occlusive disease.
Basically no. Unless you have a bruit, or a noise that the doctor can hear with his stethoscope over your neck, and if you are sure that you have no plaque elsewhere, and if you are not having symptoms related to arterial blood flow to your brain, such as a tia, stroke, etc, then you do not need an ultrastound at your age. Concentrate on controlling risk factors, such as cholesterol, blood pressure, and weight.