@yugefamily like @Eeyore123 's child, mine used SAT tests to prep for the PSAT in junior year as well as completing the PSAT practice test the college board provides. She took both tests simultaneously which was fine in her case because she had completed all of the math content that might appear on the SAT. Had that not been the case, I would have advised her to wait.
For both of my kids, we did what @lindagaf recommends - practice on practice tests under timed conditions until you reach your target score and then take the test for real.