phidauex
I don't think you would need to change anything else in OSM. Things with proper names will always have a variety of spellings around the world - I wouldn't recommend that we change "Matthew", "Mathieu", and "Mateo" to the same thing, even though they mean the same name, because a proper name can never be spelled "wrong" - however the person/thing names itself is its name.

I think the question here is fairly simple - is the proper name of the city "St. Louis" or "Saint Louis (but sometimes written as St. Louis)". Those are two very different things. In the first case, the "St." isn't an abbreviation - it is part of the proper name. In the second case, it is an abbreviation for convenience, and should be "unabbreviated" in OSM. My current knowledge about the city suggests that "St. Louis" is the proper name, but I'm open to records that might show something different.