On the Down syndrome screening test my due date was miscalculated. Does it matter?

Several test types. There are several types of testing availble, from sonogram, chromosomal testing from chorionic villus or amniocentesis and new blood tests. A small difference in fetal age will not likely invalidate all of the tests.
Not the issue. Studies show that most people don';t change their minds if test positive, so no real reason to do test unless your OB asks you.