Listen to that guy. Do it during an easy rotation, try not to do it during snowy months (weather delays) and actually prepare for the test. If you do it too early and fail, every residency program will see. If you do it too late, you won't be ranked. Fall of 4th year is good. There's not a specific month during third year to watch out for--it depends on your rotations. Would recommend taking at least after your FM, peds and IM rotations.