I'm ok with putting TkFixedFont in the config file. Only potential downside I see is that the code will look for this specific value, but people may read the config file and assume it could be changed to any Tk*Font.

I'd strongly argue against putting in a default button and/or including TkFixedFont in the list for the following reasons:

1. The config dialog should really be using the platform-specific font chooser for this anyway. (Tk 8.6 does include this dialog btw). The default dialogs don't allow anything like that, and I can't off the top of my head think of any common applications that allow you to revert back to a default.

2. I see no real advantage to being able to switch 'back' to a default. In fact, from the user perspective, there isn't conceptually a default font separate than whatever the 'actual' version of TkFixedFont is (courier, monaco, whatever). What they know is it starts with a certain font, and if they go into the dialog, it shows that font. If they change to a different font (or even back to the original font), that's what shows up next time they start the app. 

Is there actually a downside to writing out the per-user config file even if there isn't a "real" change in the font?