The only reasons to start before freshman year would be
- you were targeting one of the talent-search programs for 8th-graders, which use the SAT as a qualifier.
- you were dead set on achieving National Merit recognition
In which case additional prep might be warranted.
For high school students looking to use the SAT as an admissions test (as most do) Sophomore year makes sense to start prep because:
- Most sophomores will be learning the math needed for the SAT/ACT (Algebra II and Geometry)
- It allows you to start prepping before the intensity of Junior year sets in
If you will not be taking the math you need for the test during sophomore year, then you might want to wait, or budget some additional prep time to learn that math.