I can't comment on 50s but I have two sets of M60s. One pair is in a stereo setup in a room that has no TV, so music only. The second is in a full 5.1 setup with a VP150 center.
I am extremely happy with both setups. In the music only, stereo setup, the sounstage is excellent and I don't feel at all like I am missing a center speaker.
However what i will add, is that in a 5.1 it is nice to be able to bump up the center channel a db or two to help bring dialog to the forefront. A lot of movies are mixed in such a way that the dialog can be hard to hear compared to all the other noises. Being able to adjust this subtly can bring a BIG improvement to your listening enjoyment. Nobody likes to strain to hear dialog or having to keep the remote in hand to keep playing with the volume as it switches from loud special effects to dialog.
If movies will be a big part of it's usage, I would suggest getting whatever matching center you can afford. You can always upgrade later. If music, I wouldn't sweat it. Marks advice on a 'phantom center' will work well unless you are concerned about the people not sitting in the sweetspot. And who cares about those guys, right????
