As usual, there seem to be small but significant differences among the implementations.  Testing with the three OS X Tk implementations shows that currently:
1. Aqua Tk 8.5 and Aqua Tk 8.4 do not supply a default extension
when saving.  With patch issue4832.diff applied, they still incorrectly do not supply a default extension. (Note, Aqua Tk's do not supply a file type filter in the Save File dialog box.)
2. X11 Tk 8.5 behaves like other Unix X11 Tk's: a default based on the selected File Type filter is used, i.e. .py for "Python files, .txt for "Text file".  Applying the patch does not change that correct behavior.

I tested a variation of Guilherme's patch which supplies a defaultextension of '.py' but only for OS X (rather than Windows).  With that patch (issue4832_rev2.patch):
1. Aqua Tk 8.5 now correctly supplies a default extension of '.py' on saves if the user does not enter an extension as part of the file name.
2. Aqua Tk 8.4 behavior is unchanged: it still incorrectly does not supply an extension if the user does not.  I don't see how to fix that but the use of Aqua Tk 8.4 is diminishing.
3. X11 Tk 8.5 behavior is unchanged: it still correctly supplies a default based on the selected File Type.

Since Aqua Tk 8.5 is the major Tk for OS X these days (it has been the system default since OS X 10.6) and the patch improves things for it, I'd like to see this go in.  I did test it on a Debian Linux X11.  It should behave the same on Windows as Roger's previous patch but it would be good to test it again.

As far as documentation changes, I don't see the need to have anything more than the usual IDLE/NEWS.txt file entry.