Whether you need a rec from your school principal depends on the requirements of your MOCs. Before you go asking your principal to write you a rec, check your MOCs' websites or call their offices (if info not available on website) asking what recommendations they require. If you plan to use this as an "optional" letter of rec, make sure the principal knows you well. If he/she doesn't, you're better off getting an optional letter from someone who does know you well.
MOC packets are due in the fall. Due dates vary from one MOC to another but most have deadlines between Sep 30 and Nov 30. Thus, you don't need to get a rec until the fall, although you may want to do so now, if it is needed.
MOC packets were all sent 11/5, USMMA packet sent 11/15, USCGA packet sent 10/29.
MOC required 3 letters minimum, 1 from her Counselor, and 2 from other areas..(no problem with more)
(hence the church youth group leader and Girl Scout leader). We added her principal since we are a small school and she has witness and interacted with my daughter during all her after school and in school activities.
She had many teachers volunteer to write letters of rec. for her, so her CGA application actually had 9 letters total (I think 4 were required for CGA)! the rest all got 5 letters. Some of the the teachers have been with my daughter since middle school as a teacher or advisor so they ahve seen her in many different ways (a student, an athlete, a volunteer.. etc)
All the letters touched on different attributes so we just included them all. As some of her teachers are also her coaches, her last year's science teacher has also been her track coach, he is currently in Iraq with the reserves, her insisted he write a letter and he sent it to us sealed. (the CGA required all letters with their forms be sealed in an envelope and signed on the seal)
Be sure with MOC applications that you follow instructions. If they say to send 3 Letters of Rec, do NOT send more unless you are sure they will accept more. Each MOC has different policies and you need to check with each. Not following instructions is an easy way to get your extra letters tossed (which really isn't fair to those who spent time writing them) or, worse, getting your application tossed.
In terms of SAs, I can only speak for USNA which does accept additional letters of rec. However, they are NOT required. Also, if they don't add something that is not otherwise in your packet, don't bother. With letters from your math and English teachers, your activities list, your essay, and a BGO interview, most important stuff will already be covered. Remember, most SAs receive about 12,000 applications each year. Extra letters that don't add anything substantive are mostly a nuisance.