I think this topic varies so much from school to school that for Narps it is close to impossible to make a recommendation without knowing what specific schools they are interested in and even then it may vary from school to school such that a recommendation is difficult.
For student athletes, my experience has been the final number is all that matters. Specifically for the Ivies the academic index is a formula that uses your best super scored SAT and they will encourage you to keep taking it until you hit the required score(this happened to DD's best friend who was also in her recruiting class)
As far as when to begin taking it I prefer sooner rather than later but Vertigo is absolutely correct that your math scores may suffer a little if you have not yet taken Algebra /trig. My reasoning for sooner is the typical student hasn't completed Algebra 2 until Soph year so summer before September 1 would be first opportunity to take it. I prefer to know if I should be considering spending the significant money to pursue Ivy camps before then. As a side benefit because they use super scoring you may get a very high verbal score that will allow you to focus on the math section going forward. This happened with my DD she crushed the verbal on her first attempt at the end of her freshman year. Wound up being her highest verbal score of her 3 attempts at the test and could focus on prepping for the math exclusively after that.
No matter when you take it you should absolutely prepare for it. I think it is highly worthwhile to take a practice test under test day conditions of both the SAT and ACT to see if your child excels on one over the other as the format for each caters to different students. With this info you can begin to prep for the test most suited for you.
