I too woud choose the risks associated with an ultra-sound over the risks of no ultra-sound.
One of my friends had a little girl with a huge cyst in-utero that impacted on the size of her lungs. Because the ultra-sound picked this up they had a specialist team and operating theatre on stand-by when she was born. Now she's a happy, healthy little girl but the outcome could have been very differant had there been no fore-warning of her condition.