I would say the absolute latest should be June 18th. The key here is having a score that makes you competitive. If you are not in spring classes, take the April exam, but you better be studying from now until then. If you are in spring classes, do the June exam so you can have time off classes to dedicate at least a month to hardcore studying/practice tests, while probably doing content review during the semester. You can still get the verification process underway on June 1 while studying for the MCAT. The way to work the system is to get verified by applying to some school you probably wouldn't attend, a "throwaway." The moment your scores are in, you can submit to all of your actual schools appropriately. Your practice tests should give you a good indication of where you will end up, so some sort of list should be in the works.