When dealing with Retail DVD's in Handbrake is it better to downmix to "Stereo" or leave as Dolby Pro logic II (default) ? I know DBL is backward compatible, but i'm only using it using the m4v (Apple TV preset) with built in TV speakers
I'd leave it at Pro Logic. You won't hear a difference when playing back on a stereo system, but will get better sound should you play it back on a multi-channel system one day. Of course that assumes that the DVD had multi-channel audio to begin with. But this is moot if you use the standard audio configuration from the Apple TV preset (which generates one stereo and one separte Dolby Digital 5.1 track by default).
Just as a note, the stereo AAC track is needed if you ever want to play the file on an iOS device using the native Videos app (which can't decode AC3).
Just as a note, the stereo AAC track is needed if you ever want to play the file on an iOS device using the native Videos app (which can't decode AC3).