I disagree with that. Once it's caulked, what difference does it make?
The pan is waterproofed so if moisture does get down there, it will follow the slope to the drain.
Porcelain tile: .05% absorption rate
Urethane grout: .05% absorption rate
Silicone: Well, it's s-i-l-i-c-o-n-e
The reason I tile the pan last is it's easier to see the joints I need to cut out from the wall instead of trying to follow the bottom of the first of row of tile. Also, by doing the walls first, I'm not standing on the freshly laid floor tiles.
I know many install the 2nd course of tile, floor and then go back to the first row of tile. Seems like too much jumping around for me.