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If you shoot b/w directly, or shoot raw and convert JPGs in camera, then you'll get the camera's rendition of b/w more accurately then using a preset in post. Depends how much you care about your camera's rendition of b/w. I wouldn't go from colour JPG to b/w JPG in post though, personally - there's often a lot more latitude in the raw files that would help you get the b/w tones you want.
Question: if I want to take a b/w picture is there likely to be noticeable difference between setting the camera to b/w over converting it afterwards?