I think it really depends on why they're testing, and what you mean by preparation.
We did both SCAT and EXPLORE this year to have a baseline number. So we looked over the "what to expect" booklets, and talked about what the test process would be like, but that was it. My DD is not the kid who learns a math concept after one exposure, or who is capable of remembering things she's been told to learn by rote, so any last-minute cramming either would have artificially boosted her scores with no long-term benefit (defeating the purpose of establishing a baseline), or just would have confused her.
DD didn't make the SCAT cut in math; if she wanted to test again in June to be eligible for a CTY class, I'd prep the heck out of her. In light of her EXPLORE scores, I think she'd be fine in a CTY science class, even if it were heavy in math, and I see no point in keeping her from that just because the SCAT math section is not a good measure of her actual math and science skills.
When she gets old enough that the tests matter for anything, if she wants to prep specifically for them, that's fine by me. We're planning some assiduous after-schooling / summer-schooling because she has interest, and presumably that will boost her EXPLORE score for next year, but I see that as the equivalent of subject acceleration or tutoring in an area of interest or providing access to more-advanced books, none of which I see much objection to.
