Original Format
With the exception of doing some bass management, I like to listen to the format that is on the disc, which is generally 5.1. If the audio on the disc is in stereo, then stereo it is. I'm not too keen on matrixing sound to surround speakers if it wasn't there in the original track. My setup is currently 5.1, so 7.1 isn't yet an option, but I might have to add some speakers if 7.1 soundtracks become more common with Blu-Ray.