Would you want to have amniocentesis or chorionic villus sampling to determine the genotype of the fetus?

I think a key ethical issue behind doing any sort of test is whether you can actually do anything about the situation once you have a result. For example, testing for Huntington's disease (a fatal, incurable genetic condition that usually presents in middle age) in someone who has already been born is often controversial, because a positive result essentially means that the person has received a death sentence and can do nothing about it, which can have devastating psychological consequences over the years or decades that person still might have to live. However, it is more justified when it is someone with a known family history of Huntington's trying to decide whether to have children of their own, in which case they may be willing to face that possibility.
In the case that you are presenting, a secondary issue arises, namely are you *willing* to do anything about the result. Both amniocentesis and CVS pose definite risks to both the fetus and the mother - it's not an unjustifiable risk, but it is definitely a consideration. Down's syndrome (the most probable reason for testing given the mother's age) is also an incurable condition - basically, the only options a parent has upon receiving the result of genetic testing is whether to abort the fetus, or carry it to term. If the parents are prepared to consider abortion, then yes, the test is absolutely justifiable. However, if the parents are determined to raise the child regardless of his/her condition, then I would tend to say don't do the test - it does not influence prenatal care, nor the parents' decision, so testing would just a source of possible unnecessary harm to the mother and fetus.