I don't know why UA even promotes UDU updrades clearing for 1Ps at the 72-hour mark as mine always get waitlisted. It doesn't matter if the F class is completely empty, I get waitlisted and don't get a confirmed upgrade until I am at the gate.

With an upgraded waitlist status, should I check in online at the 24-hour mark or hold off on check-in until I get to the airport? Is there a tried-and-true formula tat seems to work best?