USMLE is very tight-lipped about how they determine their curves so it's hard to say. However I believe that each question has its own curve, which is determined around now when they tweak the question pool (which is why so many of us are waiting a while for scores). Because of that, I don't think it really matters where and when you take it. It isn't like the SAT/ACT/MCAT/etc where everyone who takes it on a single day gets the same questions, everyone gets different questions from I assume a 10,000+ question pool, and each individual is scored independently based on how everyone has previously done on that question. Hopefully that answers your question!