most people take them during there junior year after taking the PSAT, so they know what areas they need improvement on. Some people take prep classes sophmore year if they took the PSAT that year or if they know they have a weak area.
I WOULD BEGIN PRACTICING WITH THE BARRONS GUIDES IN FRESHMAN YEAR & BEFORE JUNIOR YEAR START WITH THE STANLEY KAPLAN OR PRINCETON REVIEW CLASSES. THESE CAN ONLY HELP YOU. THE MORE SELECTIVE THE SCHOOL, THE PICKIER THEY ARE ABOUT TEST SCORES. UNTIL AS SUCH TIME AS THE SAT GOES AWAY IT'S WISE TO PREPARE. THE SAT IS NOT A TEST OF HOW MUCH KNOWLEDGE YOU HAVE ACCUMULATED. IT'S TESTING WHETHER YOU CAN APPLY CONCEPTS & THAT'S WHERE IT DIFFERS FROM TYPICAL SCHOOL CLASSES.
You should take them as early as possible. Preferably start late summer before Junior year. This way you are way ahead of the game. You catch my drift?