On the M14 forum someone posted to not use any but military ammo unless you have the headspace checked.
That's a load of rubbish. The M1A is basically the civilian version of the military M14. It would make no sense for Springfield to design their rifles around military ammunition. I have shot plenty of standard Remington UMC .308 from my buddies M1A, and in fact I plan to get an M1A Scout in the near future.
By the way, check the box your rifle came in, there should be a slip of paper in there from the factory with your exact head space for that rifle.