While an ultrasound is helpful to determine your vein anatomy, it is not always necessary if you have mostly spider veins. In my office, I perform the full range of vein procedures-- including endovenous ablation, microphlebectomy, and sclerotherapy. While I always do an ultrasound on patients I suspect with larger vein issues, I do not perform ultrasound on all my patients with just spider veins.
A thorough clinical exam and history will tell me whether an ultrasound is necessary. For the most part, scattered spider veins (without any signs of larger vein disease) can be treated with sclerotherapy alone. On the other hand, if my patient has significant symptoms, prominent spider veins around the ankles, or have had previous unsuccessful treatment with sclerotherapy, I may recommend an ultrasound even if they only have spider veins on exam.
Bottomline is you should seek an experienced vein specialist who can determine if an ultrasound is necessary.
Good luck!
-Dr Mann