I went the 770 route. I use all seven channels for movies, television and often even for music.
Some folks that do a lot of stereo only listening who started with a 5.1 system and later expanded to 7.1 have expressed a preference for having 2 M200's driving their front left and right speakers, and a 755 for the center and four surrounds. That way, when listening to stereo only, their 755 remains off.
If one's music listening includes a fair amount of SACD/DVD-A, or prefers one of the surround sound modes even for stereo sources, or if one doesn't mind idle channels, then perhaps stereo only amplification becomes less important.