Ideally, it shoudl be checked during every treatment cycle, 6-7DPO. Not all places do this though...just because your P4 was okay a few months ago doesn't mean it is now.
Also, and I don't know if this is in your case or not, but part of the reason "they" say lower-stim is better is because the follicular develompent isn't compromised as much, which results (for many, not all) in higher-quality eggs and follicular development, which means it doesn't compromise P4 as much. If the follicle doesn't develop properly, it can cause P4 problems. I don't know if this is what could be happening, but something to think about perhaps. When I saw Dr. Z.hang a few weeks ago, he even told me that based on what he saw with my BBT charts and follicular development on FSh injects, most doses of FSH would compromise the quality and in turn cause P4 problems and he would'nt use them with me (but I think that is semi-SOP for him anyway). I don't know who/what to believe about some of this stuff sometimes...everyone has varying opinions on this subject.