Implementing file I/O is far from my favourite thing in the world.
Probably because I wanted it over with, I got a little ahead of myself, and tried to work in the SteamCloud functionality at the same time as the local stuff. This was ill-advised.
Suffice to say, for now, your high scores and letter grade data for each stage will save on your own machine. Your progress at large (i.e., tier gates cleared) will be pulled from Steam, if it's the more recent data, or from your local save.
Let me know if this works as I'm hoping it does :)
Now, on to the Cloud....