Not sure exact cutoff but the best advice I've heard on other forums for other competitive fields is to take it soon after submitting your app. That way your score is not automatically sent to the residencies before you see it. If you like your score, then you can send it. If you don't like your score, don't.
That's what I'm going to be forced to do regardless. My school's 3rd year doesn't end until the end of June. That makes July being the earliest I can get an ophtho elective in for the app. Doing an optho elective for my app is way more important than getting Step 2 in on time. I plan on taking Step 2 at the end of August regardless. That also gives me the choice to send it or not and I'll still have taken it soon after 3rd year before I forget all that crap. Works out well, I think. Going to doublecheck with the school advisor at some point, though.