It may be a good idea to take an SAT subject test whenever the corresponding class is completed, so that, for example, a 10th grader taking biology (and not planning to take AP biology in 11th grade) takes the biology subject test in June of that year.
You want to be a little careful about this if you are aiming for top colleges. A few colleges require EVERY score to be sent if the test was taken. And the curve is quite steep on the subject tests, I think (so you might think that 700 on Math II is just fine, but in fact it would be 52nd percentile in 2012). So you want to be careful that your kid has not just taken the class, but studied for the test and taken some practice tests with decent scores before they take a SAT II. You don't want your kid showing a bunch of retakes on the tests and some low scores that are never retaken for those schools that require all scores.
Just as an aside, for whatever reason my kid always had trouble scoring well on the Biology test. Maybe because there is so darned much material in Bio...
