phidauex
That is a tricky one - the charter for the city gives it as "The City of St. Louis" in the very first sentence. Living there, I rarely saw it spelled out as Saint (though apparently you are correct about the city contact info page). Some institutions, like Saint Louis University don't abbreviate, but that is because I think they are named after the actual saint as much as they are the city itself.

I'd vote to keep it as St. Louis given the history and charter, but that other uses of Saint/St. around town would lean toward Saint.