It is almost always better to convert to black and white in post, because you have much more control over the process. If you use the in-camera conversion, you get it the way it converts. If you shoot in color, then you have many different ways in post to convert the image.
Here is an overview for B+W conversion (in GIMP).
Another way to think about it:
Suppose that you are told that you can have one free item from a candy counter. In camera conversion is like asking for the one in front without actually looking. Doing your own conversion is like looking through the selection and picking your favorite.
RAW vs JPEG
I actually prefer to shoot RAW, so I do select black and white in camera, but that is because in RAW mode, the only difference is the mode that is listed in the EXIF data. The actual image data is exactly the same as if I had selected color. If I were shooting JPEG I would probably never select monochrome because I don't want to lose control of the conversion.