Data point (for 9:20 flight)
I checked in at 9:20:02 and got A46.
I received an email from AirlineCheckins.com at 9:35 followed by a boarding pass email from WN.
So, probably not good to rely on them to get a low boarding number. Unsure if I'll bother using it again unless I'm concerned I may oversleep the check-in window or be stuck somewhere without Internet or something.