Some will hear before, some will hear after. Keep in mind though that Summer Seminar is a recruiting/publicity tool, and that admission is not necessarily based on who is the most qualified. Which means that the applicant from rural south eastern Montana who sends in an application in early February is likely to hear much sooner (and be accepted) than the applicant who applies in early February, but lives in NoVa.
If you are coming from an area that sends lots of students to the academy, the admissions office may not see any need to send you to SS, and may be waiting to see how the geographic distribution of applicants look before deciding. <- but now I'm just speculating.... As for your original question, yes, technically the longer you wait to apply to "rolling admissions" the less likely you are to be accepted. But also remember that they are not going to give slots to the first 2000 (not sure of exact #) "qualified" applicants. With this large of an applicant pool, rolling admissions does not always mean rolling admissions in the traditional sense.
As anecdotal evidence; I submitting my application a week after it opened, was wait listed on April 15 and then got in off the wait list in the middle of May.
I would advise not to sweat it. Enjoy the rest of your junior year, you're about to be a senior!
