[Otto]

The short answer is "you can." The reason to use analog I/O is to be able to measure your soundcard's response, so that you can account for it in the measurement. You do that via the soundcard cal in REW.

I have measured my soundcard using analog I/O. I have then used a digital input and an analog output, and measured much the same response, so that tells me that my soundcard's response is coming mostly from the input of the soundcard rather than the output.

Poke around at it a little from both sides and see what works for you. Certainly, if you just want to get things going and see how they work, a digital input to your system is a viable option.