My doctor heard no bruits in my carotid arteries. could there be any benefit, still, to an ultrasound of them?

Yes: Yes, there can be carotid stenosis without audible bruits, so if your doctor is looking for stenosis, or characterization of atheromatous plaque, carotid ultrasound could be useful.

Yes, depends on hx: The stethoscope is a very blunt/insensitive test. Plaque build up may be at the beginning stages, in which case u might see it on ultrasound (US) but not hear it - if atherosclerosis runs in ur family, the US wd b useful. If the plaque is very advanced, nearly blocking blood flow, then the same: it can be seen, but not heard. Again, the US is helpful as it cd validate symptoms of a stroke or TIA.