NW agents can tell if you are onboard or not when they do the upgrading at the last minute. It is up to the gate agents working the flight as to whether they want to upgrade someone who is onboard or someone who isn't shown as boarding yet. Most agents who are lazy and just want to close the doors will do the latter and then it is best to stay off the aircraft if that is the case so you can get upgraded. Ultimately, it really relies 100% on the gate agent. I have noticied that in MEM they clear the list right before boarding (30mins prior) and in DTW they will wait until 10-15 mins to clear the list. I have never been waitlisted in MSP.