It's possible, though personally I wouldn't like it (but I don't use IDLE, so I guess that makes the difference). I'd prefer to be able to list them all under the Open With menu, since that also allows users to easily make any of them the default if that's what they'd like. Unfortunately, to do that we need to start shipping an EXE launcher, probably with a unique name (e.g. idle35-32.exe).

Alternatively, if IDLE provided a selection of installed versions within the program, then it would make sense to always open with the most recent version (or more likely, the most recently installed version, due to how file associations have to be set up when you don't have separate .EXEs).

Currently the 3.5 installer doesn't add any commands for IDLE, so when I fix that I'll include the version information. It may require that the py launcher is also installed, since the launcher owns the file association.