Taking the Subject Test and taking the general SAT is different. A Subject Test should be taken at the end of the year in which you complete the test content, She should take a quick review of her Algebra II, Trig and Pre-Calc and take the Math Level II Subject Test this June.
While the SAT has changed, I would not recommend taking it "early and often". Some colleges do require that you send all scores, and I do not know that they examine them so closely that they realize which ones are "early sittings" and which ones are later. Does Yale really look carefully at the test scores of 32,000 applications to know when you took the test?
I've heard the tale before - my kid is really advanced at math and he "forgot" the math on the SAT/ACT. I'm not buying it. The kids I knew who were really advanced - as in taking college courses in 9th grade because they had taken Multi-Variable Calc and Linear Algebra already, and the HS doesn't offer courses beyond that - received perfect scores on the SAT/ACT math. This includes a kid who qualified for USAMO every year from 8th grade on.
If she is concerned with forgetting the math, have her help at a HS peer tutoring center or similar. Will help her remember and she'll have an EC/volunteer item for college apps (and she'll help a struggling classmate).
One of mine did take the SAT in spring of 10th grade, but that was in part driven by wanting to have a score for college coaches to consider at showcase tournaments.
