Do you mean a venous ultrasound? I've had several, and it varies slightly by institution/technician. The usual course is check from the neck (jugular vein) down to the elbow. Some will continue all the way down to check forearm veins as well. The most thorough test I had checked veins and arteries in arms down and arms up positions. It seems kind of pointless to me to only check when arms are down, unless there is concern over a clot, of course.

ETA: It's important to have a tech experienced with testing for TOS. The collarbone blocks part of the subclavian vein so it's necessary to manipulate the transducer to a degree.