Is it necessary to do an amnio?

No. It is a personal decision. Some patients want to know with 100% certainty whether or not the baby has a chromosomal problem and would terminate a pregnancy if it had such a problem. Some patients do not want to risk losing a normal pregnancy because of the amnio and would not terminate the pregnancy.
Not generally. There are indications for performing an amniocentesis. Certainly not all pregnancies warrant that procedure.