Damado, that is a good question, and one I really don't have an accurate answer for. I do know that your API NSS grade counts for 15% of your Primary NSS:
15% API
15% Primary Ground School (Systems Test, Metro Test, INAV Test)
70% Monkey Skills (Flying..)
I dimly recall being told that the PRT at API was something like 5% of our API grade, but I could be mistaken. Maybe someone else who has just been through API could fill that in?
Bully, as for the importance of a good score for Service Selection... well, you should try and get the best all round package that you can put together, and scraping along at the minimums might not be the best idea, espcially if it comes down to your package against someone else's and your PRT/PFT scores are brought up... just a thought. I personally don't know the exact weighting of each of the package components to get into Aviation, best resource should be your Recruiter/NROTC/Academy...