If you're over the age of 35 I believe it is recommended, even mandatory in some places. Otherwise no, you don't need to get it done. I didn't do any prenatal screenings because I knew I'd never choose to abort my baby if they had Down Syndrome or something.
Also, those tests only give you an estimated guess on the chances your baby could have a genetic anomoly and often give false positives which can cause the parents months of grief only to give birth to a perfectly healthy baby.
I say don't worry about it!
I don't think it would hurt not to get it done they are basically just looking at the fluid between the neck and the spine and giving you your numbers for whether you are at risk or not. Atleast that is what they did at my appt and they make sure that everything is growing the way that it should and that the hb is normal..
You dont have to have this screeing done, So yes it is Ok. If you would really like to have this u/s done I would call your Ob and see if there is another place you can go and have it done at.....