Try to get as much of your testing done as you can during your junior year. Winter is a great time to take some of the tests since you will be really busy during spring race season. If you are a sophomore, think about taking an SAT II test in one of your strong subjects. Talk to your teacher about the possibility of taking the SATII in that subject during the spring of sophomore year. This will ease the testing pressure next year.
The first thing coaches ask for after your height, weight and erg score are GPA and SAT/ACT scores. The bulk of recruiting occurs with early applications so waiting to take tests during the fall of senior year can slow down the process. Be sure to take time to prep for the tests since you want to do your best and have good results to show both the coaches and admissions. The work that you do now will have an large impact on which coaches may be interested in recruiting you as well as on which schools you may ultimately gain admission.