[ghaff]

In the US--I can't speak to CLEAR--but even with TSA Pre you have to show your ID. (I'm not sure they've even looked at my boarding pass when I've flown recently. I assume the computers are correlating my ID with the flight reservation in the system.)

If I can conveniently print out a boarding pass either at home or at the airport I tend to do so. That way I'm not fiddling with my phone when I don't need to.

(I think I've seen it the other way around in London at least.)