For any FT near the top of UA upgrade standby list, do not board early. Just wait near the podium until you either get a boarding pass or are told that FC cabin is full. This applies to top two if App says that FC is booked full and checked-in full. If there is one or two seats available, the top three or four should wait to board. I trust that most GAs perform their duties well, but I am wishful for the last upgrade seat before the door is closed.
BTW, this type of thread will not appear if every FT follows this practice!
Cheers!