Counter to the opinions above…both my kids took their tests the spring/summer between Sophomore and Junior year. If you have had math up through precalc - and did well - you can handle the math on the tests. However, I would definitely advise you to prepare. Both kids did a number of practice tests, focused on correcting their weak areas and then tested again. Practice tests were timed in simulated test conditions (i.e. morning, no breaks other than would be in the tests, etc.)
Both kids were one and done. This was a big advantage as they both had tough junior year schedules. In addition, they were able to provide test results to college coaches, which helped a little in the recruiting process.
Given Covid and all of this year’s unknowns, I’m thankful that my S21 has a good test score in hand. Even though many schools are test optional, it helps to know where his scores would put him as he goes through and attempts to make his college lists.