I actually had the venogram several weeks ago. I'm not good with procedures generally (I get very anxious and tend to faint), but it wasn't too bad at all. I asked for a little bit of a conscious sedative and was glad I did. I was also given lidocaine as a topic analgesic for where they put in the needle, and barely felt anything. They took pictures while my arm was by my side and at 90 degrees out with my elbow bent up. My arm was sore for several days after (and I wasn't allowed to get the bandaid wet for 2-3 days or lift anything for that time), but otherwise I was fine.
This is actually how I was diagnosed with venous TOS. I first was diagnosed with neurogenic, and then my nerve symptoms got significantly better but my arm would swell (minor and no clot) and change color. They were concerned about a clot, which I luckily didn't have, but the doctor showed me where the blood is supposed to flow and how collateral veins had developed because the vein narrowed to the point of near occlusion when my arm was raised. They were also able to see scarring of the vein.
The venogram is considered the "gold standard" for looking at blood flow and clots- much better than a contrast MRI, or so I was told, and I would definitely recommend having it, at least for your peace of mind.
It's still hard for me to accept and I'm worried, but I'm going in for surgery on August 21. I think it's better to know whether or not you have a high clot risk and take care of it than to wait until you actually get a clot...