I agree with get there early especially on NGO-DTW, I took my first trip ever on NW(no status with anyone at the time) and on the return I was 5th to check in and the supervisor ran over and begged me to take the op-up and a voucher for a slight re-route. And being polite never hurts.

And your logic for taking the later flight may be flawed in that if the earlier flight is oversold they may op up them onto your flight.