there is no proof either way that there is risk/harm to the bub. BUT I know that bubs are aware of the u/s...atleast with older ones, you up the probe on and they kick it and react to it. No one knows though whether this reaction is good or bad.
Up to you though, if it would ease your mind then might be better than you stressing. like Kass said, cost would be a thing. and I guess having a care provider that is happy to do it. =)