All that I can tell you for sure (haven't actually read the new requirements) is that I had to turn in all my receipts for food, gas, and lodging when I filled out my travel claim forms (you will fill this out at OCS). However, if I remember correctly, I was given the per diem rate even though my receipts didn't necessarily add up to the per diem rate. For example, if I was traveling from California to Colorado on PCS orders, I think I was getting $125 a day for per diem regardless of how much I actually spent. I can tell you that it does work slightly different for TDY orders (those are the type of orders you will have when you travel to OCS).
Sorry to give you such a vague answer. Hopefully, someone who has done it more recently can shed more light.
-Jason