We play that whoever has the highest double puts it in the center of the game piece and then also goes first. Then they have, conceivably, the first shot at starting the mexican train as well, so it's a big advantage going first.
So if no one has the 12, it's the 11, then 10 and so on. Then everyone has to rearrange their tiles if it isn't 12...lol.
I know some people play with the descending start tile, but we always start with 12 if someone has it.