It depends on what part of the country you live in. They told me this when I talked to them on the phone. Call and ask.
But frankly, it sounds like you will probably get the LIBOR + 3.75 rate regardless. One the other thread we pretty much figured out that a person had to have at least in the mid-700's to get the highest rate. And I doubt, although I could be wrong, that 600 is low enough to make them require that you have the LIBOR + 6.75 rate with a co-signer. I think you have to have pretty poor credit for that.