Just my opinion but in B&W for individual images (competitions) I usually pick the mount to compliment the tonal range of the image and how I want it too appear to the viewer.
A black mount will emphasis lighter tones of a image while a white mount will emphasis the darker tones..try a black and a white mount on the same image and see the difference it makes to the perceived tonal range
For a set of images for wall display then you have no choice but to use the same coloured mount.