You will thank yourself if you take it sooner (same with CS!). The old wisdom of taking it later if your step 1 score is high still works for some people. However, there are many programs that like to see your scores prior to offering interviews (i.e. it's "suggested") and a few that require seeing your CK score to rank you. Given that things are getting more and more competitive each year, our advisor strongly recommended we take it before subI season.