Quick Answer
The safest rule is simple: check the official AAMC participating-schools page, MSAR, and each school's admissions website before you decide whether and when to take AAMC PREview. A school that requires PREview may not consider your file complete until the score arrives. A school that recommends PREview may still allow you to submit and be reviewed without a score. A school that is exploring PREview may see scores for research or evaluation but not use them to evaluate current applicants.
Because those categories have different timing consequences, applicants should not treat PREview as universally required or universally optional. The same score can matter differently depending on the school reviewing it.