If I'm checking bags and/or the line looks short, I'll do it at checkin counter.
Next I ask if they can do it at the President's Club (even if i did it at checkin counter) to make sure it's done right.
If I didn't make it to the Club, i'll do it at the gate (again, even if i did it at the checkin counter)

WHen your companion is an elite, it's good to look at their position 'before' and 'after' to see they changed.

When companion is an elite, i like to say smt like "i would like to put my companion on upgrade list, as 'platinum companion.' I know She/he is on the list as silver; being listed as platinum companion will move her/him up the list."
This reduces chance they'll look, see the companion is on the list (at last slot, as silver on cheap ticket...), and do nothing.