It's really a difference between students going to US schools and those going to non-US schools.
All IMGs MUST have taken and passed step 1, CS, and CK before rank-order list time (Feb. 21st). No exceptions...well, unless you as prematching
US medical students can take step 2's whenever they like. If step 1 is good enough for you, don't risk getting a poor step 2 score...wait til after the match (or rank list submission).