Keg,
You're right stating that Airbus stipulates doing it after line up clearance but the company has given us approval in FCOM to do it prior, so that there is no further distractions. Keep in mind Airbus also says your tray table goes away after a line up clearance is received.
That's particularly useful in the days of ATC asking 'confirm ready immediate? After having called ready.
I also make the point of matching the movement area guidance sign with what we've written on the TOLD card SE there is less potential for error.