When my child looked at BS last year, I very specifically remember that at least a few schools said that they take the math SSAT score into account when determining math placement, in addition to the math placement test and the child’s prior math coursework. I didn’t specifically ask about this, so I don’t know whether many schools do this or most don’t, or even how much the score matters in placement, but at least a few schools said the math SSAT score is used after admissions that way.
So if your math SSAT score is uncharacteristically lower than you expect, based on practice tests, even if your score is overall fine, you may want to think about whether you want to retake the test.
As others have said, great SSAT scores are not going to be the thing that gets you in, or even tips you in. My child knows kids with 99th percentile SSAT scores who did not get accepted to BS, both competitive and not-so-competitive ones. And poor SSAT scores might keep you out (not will - might), even with an otherwise great application, because the school might worry about whether you can handle the academic workload if your SSAT scores are poor.
So I’d recommend that instead of focusing on a particular score, do some reasonable amount of prep, and do the best you can do. Don’t kill yourself for SSAT prep when you could be spending the time in other ways, but don’t walk in cold with no idea of the format or types of questions on the test either (sounds obvious, but we know someone who did this, and misunderstood some of the instructions and bombed a whole section).