I guess it depends what you're doing, but I'd say that the most common surround setup is 5.1 - don't quote me on that, and seeing as Dolby Digital is used on all DVDs, that might be the may popular format. That's a wild stab in the dark though.
A surround track played on a stereo system will not send every signal to the stereo speakers, it will just miss out the signals intended for the back etc, else the likes of surround sound automated panning would completely fail.
Which speaker signals are sent to is determined during mixing.