True.
Take the ACT or SAT (at least PSAT) well before that so they know you are worth watching and in case you need to get serious about test prep to get your scores up. Get seen by the Ivies the summer before your junior year with decisions happening the next year give or take. If by Ivy types you are including the Williams, Hamilton, Amherst etc, then yeah, they get to the buffet line after the Ivy League schools are finished on average. Not that a Williams isn't as hard or harder to get into than an Ivy. I know a former Ivy coach. I'll ask her about the details of their recruiting calendar and confirm.
Mark - although nothing can be official until the admissions department sends out an acceptance in the Ivy League (The Ivies do not use/recognize NLIs), most Ivy schools are still finishing their '17's and just starting to look at '18's.