When shooting in Black and White with the DVX 100, is it better to set the camera to shoot in B&W (can this be done?) or is it better to make the footage B&W in post?

Better is post (I don't think you can do it in cam anyway).

Per post, try using RGB channel combining rather than saturation for better results.

The downside to doing in post is the rendering time. However, shooting in color allows you to use the video for other purposes when color may be required. So the good advice is to keep in color and change it in post unless you are absolutely sure you will never need the original colors.

It is also true that you will have better control of manipulating b/w in post. You can tweak in some other effects too.