Whether this makes sense depends on the subjects the student is taking. For example, if the student wants to take a Subject Test in a foreign language and will be continuing the study of that language in grade 11, it makes sense to wait a year longer before taking that Subject Test.
My son did exceptionally well in 10th grade honors biology. He asked his guidance counselor whether it would be a good idea to take the Biology Subject Test at the end of the year. The counselor said “no way” on the grounds that he would be competing with people who had completed AP Biology and therefore had two years of preparation instead of one.